[SOLVED] How do I make an entity that, when using no items, gives a potion effect every melee hit?

Use melee AI task, set condition procedure that always returns true and before returning true, give the potion in procedures

I suggest you check our tutorials collection playlist on our YouTube channel which contains many examples and tutorials that can help you get started with MCreator: https://www.youtube.com/playlist?list=PLAeL-oIFIEngE6jRgFYeFMfuj8WQsO3Ei

Procedure that returns logic value should be explained in one of tutorials I linked

Glad to hear you figured it out!