Go to our developers corner section of the forums, then, in the search box near top of screen, search for "openURL"
There are many discussions regarding 'custom' mord type page commands, perhaps one of them will lead you on your way
If not, ask in one of the discussions that at least is the closest to what you want.
Our own ULX motd code allows setting of a external website, and indeed uses the OpenURL command within confines of a set size.