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.