I don't have an account, but if I may, I also vote for the third option (trigger warnings at the top of the prompt and/or fill)! I liked erinptah's idea about having a specific list of warnings. Borrowing somewhat from AO3, perhaps they could be: graphic violence, rape/non-con, incest, suicide, character death. In the case of rarer things like "pissplay" or what have you, if it's not obvious from the prompt, authors could also have the option of using their own judgment to tag anything else they want to warn for.