>>8667000
The tag for it on Danbooru is "reverse fellatio" https://danbooru.donmai.us/wiki_pages/reverse_fellatio

Unfortunately, it seems like that has a lot of tag bleed with other reverse poses like "reverse cowgirl" and bleeds into normal fellatio too. I couldn't get it to work consistently with just prompts.

So I cheated and grabbed a LORA for it: https://civitai.com/models/543154/reverse-fellatio-lora-or-ponyxl-and-illustrious . That's the most popular one and it seems to work well.