I have added the documentation for CharacterUseSkill. The skill name comes from the stats editor. You may want to read through the tutorial on skill creation/modification to get familiar with the stats editor as far as it is related to skills.

Note that, as explained on the Osiris API start page, you can see the full definitions of all Osiris calls in the editor. Then you can also see all parameter names, which is more useful than just their types (which you, unfortunately, get in case there are overloads).