Yeah, I...don't know, either. I guess it depends on whether you feel like you want to/need to discuss fandom with your therapist or whether you can be nonspecific about the context in which you're writing tens of thousands of words.

But the thing about mentioning it at the first meeting is that you haven't yet invested anything into the therapeutic relationship, either. So if the therapist seems judgemental or weird about it, thanks but no thanks, you look for another one. (Therapist shopping as an activity does suck, but IME it sucks regardless, since you can't always predict bad fits.)