Where do I put a trigger/content warning?

Not sure if this has been asked before, but I’m planning to self-publish my first book on Amazon eventually, and the story hits on some heavy themes. So, I feel like I should add a trigger warning somewhere, but I’m not sure where. Should it be on/in the book, like back cover or a page in front of the book, or just on the description page? Both? I'm not sure. Should I mention it in posts I put up on my socials promoting the book once it's out?