In British English, only the first is correct and the second is utterly incorrect. I have heard the second in American English. I'm not sure how standard it is.

Hello, Pelin 😃 I need to urgently clarify something I recently said/wrote here: When I said the second sentence I was taking the first sentence to be: 1) Someone just came here without advance notice. 2) I wish you had let me know before you came. Correct 😃 3) I wish you " have" let me know before you came. 🫢😦🙄Incorrect!/wrong. This last sentence sounds awkward and unnatural to me as a native English speaker! Writing this as a positive request instead of a complaint, you can say: Please let me know before you come next time. Or Please let me know beforehand/in advance, next time you decide to come. I hope this helps you 😃

When I said the second sentence I was taking

I would do the first one, but both work

Hello Pelin. 😀 Your second sentence is correct.😀 You can also say: ● Someone just came without letting me know beforehand. ● Someone just arrived without warning. ● Someone just came/arrived without prior notice. I hope this helps you 😀