reference toURI scheme - Wikipedia
Register a Custom URL Protocol Handler

Generic syntax

<scheme name> : <hierarchical part> [ ? <query> ] [ # <fragment> ]

RFC 3986

  foo://username:password@example.com:8042/over/there/index.dtb;type=animal?name=ferret#nose
  \ /   \________________/\_________/ \__/\_________/ \___/ \_/ \_________/ \_________/ \__/
   |           |               |        |     |         |     |       |            |     |
scheme     userinfo         hostname  port  path  filename extension parameter(s) query fragment
   |    \_______________________________/
   |                authority
   |   ________________________
  / \ /                        \
  urn:example:animal:ferret:nose

common schemes

Scheme	Purpose	Defined by	General format	Notes
dict:	Dictionary service protocol	RFC 2229	dict://<user>;<auth>@<host>:<port>/d:<word>:<database>:<n> dict://<user>;<auth>@<host>:<port>/m:<word>:<database>:<strat>:<n>	refer to definitions or word lists available using the DICT protocol
dns:	Domain Name System	RFC 4501	dns:[//<host>[:<port>]/]<dnsname>[?<dnsquery>] examples: dns:example?TYPE=A;CLASS=IN dns://192.168.1.1/ftp.example.org?type=A	designates a DNS resource record set, referenced by domain name, class, type, and, optionally, the authority
fax:	Used for telefacsimile numbers	RFC 2806	fax:<phonenumber>	Seems to be deprecated in RFC 3966 in favour of tel:
file:	Addressing files on local or network file systems	RFC 1738	generic syntax (often appears as file:///path, the 3rd '/' is the final delimiter when no host (authority) is specified between)	Unusual in not being bound to any network protocol, and not usable in an Internet context.
ftp:	FTP resources	RFC 1738	generic syntax(ftp://user:pass@server )
go:	Common Name Resolution Protocol	RFC 3368	go://[<host>]?[<common-name>]*[;<attribute>=[<type>,]<value>] or go:<common-name>*[;<attribute>=[<type>,]<value>]
http:	HTTP resources	RFC 2616	generic syntax
https:	HTTP connections secured using SSL/TLS	RFC 2817	generic syntax
im:	Instant messaging protocol	RFC 3860 RFC 4622	im:<username>[@<host>]	Works as xmpp: URI for single user chat sessions.
ldap:	LDAP directory request	RFC 2255 RFC 4516	ldap://[<host>[:<port>]][/<dn> [?[<attributes>][?[<scope>][?[<filter>][?<extensions>]]]]] example: ldap://ldap1.example.net:6666/o=University%20of%20Michigan, c=US??sub?(cn=Babs%20Jensen)
mailto:	SMTP e-mail addresses and default content	RFC 2368	mailto:<address>[?<header1>=<value1>[&<header2>=<value2>]] example: mailto:jsmith@example.com?subject=A%20Test&body=My%20idea%20is%3A%20%0A	Headers are optional, but often include subject=; body= can be used to pre-fill the body of the message.
news:	(Usenet) newsgroups and postings	RFC 1738	news:<newsgroupname> or news:<message-id>	References a particular resource, regardless of location.
pop:	Accessing mailbox through POP3	RFC 2384	pop://[<user>[;AUTH=<auth>]@]<host>[:<port>]
sip:	Used with Session Initiation Protocol (SIP)	RFC 3969 RFC 3261	sip:<user>[:<password>]@<host>[:<port>][;<uri-parameters>][?<headers>] examples: sip:alice@atlanta.com?subject=project%20x&priority=urgent sip:+1-212-555-1212:1234@gateway.com;user=phone
sips:	Secure equivalent of sip:	RFC 3969 RFC 3261	sips:<user>[:<password>]@<host>[:<port>][;<uri-parameters>][?<headers>]
snmp:	Simple Network Management Protocol	RFC 4088	snmp://[user@]host[:port][/[<context>[;<contextEngineID>]][/<oid>]] examples: snmp://example.com//1.3.6.1.2.1.1.3+ snmp://tester5@example.com:8161/bridge1;800002b804616263
soap.beep: soap.beeps:		RFC 3288
telnet:	Used with telnet	RFC 4248	telnet://<user>:<password>@<host>[:<port>/]

Scheme	Purpose	Defined by	General format	Notes
about:	Displaying product information and internal information	Un-standardised about:blank is commonly used to display a blank page.		Widely used by web browsers, sometimes even providing interactive resources. The Opera web browser uses opera: instead.
aim:	Controlling AOL Instant Messenger.	AOL	aim:<function>?<parameters>	Functions include goim, addbuddy, and buddyicon.
callto:	Launching Skype call (+And in Hungary the KLIP Software call too) (unofficial; see also skype:)	Skype	callto:<screenname> or callto:<phonenumber> [2]	Introduced with Microsoft NetMeeting. Works with current version of Skype with Firefox, Internet Explorer and Safari
chrome:	Specifies user interfaces built using XUL in Mozilla-based browsers.	Mozilla	chrome://<package>/<section>/<path> (Where <section> is either "content", "skin" or "locale")	Works only in Mozilla-based browsers such as Firefox, SeaMonkey and Netscape.
cvs:	Provides a link to a Concurrent Versions System (CVS) Repository	Concurrent Versions System	cvs://<method:logindetails>@<repository>/<modulepath>;[date=date to retrieve | tag=tag to retrieve]
ed2k:	Resources available using the eDonkey2000 network	eDonkey2000	ed2k://|file|<filename>|<size of file>|<hash of file>|/ or ed2k://|server|<host>|<port>|/	Links to servers are also possible, as are additional parameters. Official documentation from eDonkey2000 website at the Internet Archive Wayback Machine
feed:	web feed subscription		feed:<absolute_uri> or feed://<hierarchical part> examples: feed://example.com/rss.xml feed:https://example.com/rss.xml	See Feed: URI scheme for a detailed overview of common implementations, supported software, and critics.
jar:	Compressed archive member	Java API	jar:<url>!/[<entry>]	Works for any ZIP based file.
mms:	Windows streaming media		mms://<host>:<port>/<path>	Used by Windows Media Player to stream audio and/or video.
msnim:	Adding a contact, or starting a conversation in Windows Live Messenger	Windows Live Messenger	Add a contact to the buddy list msnim:add?contact=nada@nowhere.com Start a conversation with a contact msnim:chat?contact=nada@nowhere.com Start a voice conversation with a contact msnim:voice?contact=nada@nowhere.com Start a video conversation with a contact msnim:video?contact=nada@nowhere.com	Can be invoked from a web page or via a run command or an ie browser URL (won't work with firefox 2.0.0.8). For web pages use this HTML: <a href="chat?contact=nada@nowhere.com">Click to chat!</a>
notes:	Open a Lotus Notes document or database	Lotus Notes	notes://<address>	Used by IBM Lotus Notes to refer to documents and databases stored within the Lotus Notes system. When clicked in a browser on a computer with Lotus Notes client installed, Notes will open the document link as if a Notes DocLink were clicked within Notes.
secondlife:	Open the Map floater in Second Life application to teleport the resident to the location.	Linden Lab	secondlife://<region name>/<x position>/<y position>/<z position>	Used by SLurl.com. Knowledge base article.
skype:	Launching Skype call (official; see also callto:)	Skype	skype:<username|phonenumber>[?[add|call|chat|sendfile|userinfo]]	Official documentation from Skype website.
sms:	Interact with SMS capable devices for composing and sending messages.	IETF draft	sms:<phone number>?<action>	Should be used as a subset to the tel: schema.[citation needed]
steam:	Interact with Steam: install apps, purchase games, run games, etc.	Steam, Valve Corporation	steam:<command line arguments> or steam://<action>/<id, addon, IP, hostname, etc.>	Official documentation from Valve Developer Community website

URL Protocol Handler

[HKEY_CLASSES_ROOT]
  [runapp]
     (Default) = "URL:RunApp Protocol Handler"
     URL Protocol = ""
     [DefaultIcon]
       (Default) = "c:\whatever\RunAppUrlProtocol.exe”
     [shell]
       [open]
          [command]
            (Default) = "c:\whatever\RunAppUrlProtocol.exe "%1""

After installing the example RunAppUrlProtocol app, you can put links on web pages like runapp://calc, runapp://excel, or runapp://docs that will trigger this app, look up the keyword, and execute the app.