how to make player add its own effects? what EXT to add?

I am fairly new to PF and I am learning FGC . What EXT to add so players can code its won effects or how to code effects into spells no matter if you are fighter or matial class? ty in advance

This is in the standard product no need for an extension.

Don’t think of the spells section of the actions tab as just for spells. Add a "spell class", then call it "Abilities" or something like that, then add a 1 to the level 1entry under the spell class details so that a spell header opens. Then you can add new "spells" to that list and attach actions to them - but they’re not spells.