Mistress for birthday

So my partner asked me what do I want for my birthday and I don't know how to go about telling her I'd love to have a session with a mistress!

Now my partner knows I'm submissive and sissy but she is very much a sub as well so she doesn't Dom me and I only get to explore my sissy side when I'm on my own. She has not really been super excited about someone else controlling me but its got more promising as the years have gone one.

I'd really love for some help on how to ask this question!