Dominate is a verb. It is a thing you do. Preferably with gusto. And choking sounds.
Dominant is an adjective that describes something about you or another person-ish noun. If you want to use it as a kinky term-of-art, then it can also function as a noun itself.
Jackass is someone who cannot distinguish between the two. Example: And then the jackass said, “I am a dominate male looking for slaves!”
TL;DR: if someone tells you (s)he’s a dominut, believe it.