After you play an action spell im a showdown you have to pass priority before the opponent can respond right

Yes — your opponent can't respond until you pass priority.

When you play an action in a showdown, the chain starts and you get priority first. You may hold it and add Reactions on top of your own spell if you want. Your opponent only gets to respond after you pass priority (FAQ 4908, Rule 338.1.a.5).

One caveat: passing priority isn't a one-way door — after you pass, your opponent gets priority, but if they play something, priority comes back to you to respond, and so on until both players pass in sequence (Rule 339).