Ask to DM, or invite them to DM you.
Also, make sure you actually establish a rapport. A couple of comments back and forth in a sub is premature. If you see them commenting regularly on things, then engage there, over a period of time. Let them get a feel for you
Bear in mind that reddit is not usually a good place to find a partner. I say usually because I actually met my fiancée on reddit...
We're very much the exception rather than the rule though
Good luck to you!
The only way you're getting content this specific is to ask someone to make it for you
I'd start by asking fetish content creators. I'm sure you already know this, but you ain't getting this shit for free