Sorry, it was more than 20 years ago when I wrote my last Arexx scripts and at that time I still had a big Arexx manual from OS 2.x, but I don't have that anymore and my memories are lossy, so maybe I'm wrong ...
Update: I can't remember anymore what I once used to get access to library functions with Arexx, but there are several solutions available on Aminet, even for reading icon tooltypes. Try some of the following examples ...
http://aminet.net/package/util/libs/rxwb
http://aminet.net/package/util/rexx/rmh
http://aminet.net/package/util/rexx/royalbridge11
http://aminet.net/package/util/rexx/Rxgen