How do I open a generic URL from AppleScript?
If myscheme is properly registered in the Launch Services Database to open a particular application, then you can use the open location command, which: "Opens a URL with the appropriate program."

Use as in the following examples:

set theURL to "myscheme://a/b/c"

open location theURL


Or directly:

open location "myscheme://a/b/c"


Have a look at the AppleScript Language Guide and more specifically, the open location command.