Submissive won't actually tell me what she wants

So here's where I'm at. My gf and I are polyamorous and her other gf is living with her for a few months. Meta (gf's gf) suggested a threesome and it took me a while to decide but yeah, I'm down for that.

Gf and I have a very intense BDSM type dynamic going on, and it's a lot of fun. Meta and gf also do a lot of BDSM together but I don't really know the specifics of what they do together.

Meta messaged me asking me to dominate her with gf, which i said I would, but I also asked her to be more specific about the kinds of domination she wants. I mean, that could be petting her on the head and calling her a "good girl" while we fuck her or, y'know, plastic wrap bondage. I need more specifics here.

She got, I think flustered but possibly just distracted, and didn't reply for a bit. We talked again a few days later and I brought it up again, that I'm down to domme her but I need to know what she actually likes and wants me to do, and this time I gave her some examples of stuff I've done in the past to see if any of them click. Again, flustered or distracted and no response.

So skip to yesterday, we're hanging out in the park and making out and stuff and the conversation goes a particular way and I wind up explaining how to choke someone properly and she gets very excited and we start talking about basically threesome logistics.

But... She still won't tell me what she wants! I think maybe she's just very shy about expressing her desires, or maybe there's like One Thing In Particular ™️ that she really wants but is embarrassed to ask for or something and I don't know how to navigate this. I'm very open-minded and I'd be down for almost anything as long as it's ethical, legal, and mostly safe, but I just can't figure out how to get her to tell me what she wants me to actually do while dominating her. How should I navigate this?