Download the expanded sex mod and see how it is done there.
Viewing post in Modding help
The method of the Expanded Sex mod is rather old fashioned and clunky. Aric's Expansion GitHub version uses the modern method. I recommend using the patching section of the modding guide to put the new sex action files directly into the game folder ".../files/scripts/actions". The path in your mod would simply be "...YOUR_MOD/patch/files/scripts/actions/YOUR_FILE.gd".