Let's go for
A dream scenario, as there are... some...
So, it would be a first time, with a fairly new partner. Not a first time for tickling her at all, obviously, but the first time it goes as far as that with her, beyond short, flirty tickle fights and random making out tickles.
We would be dating, probably a couple of dates since the first one, spending an evening at my or her place. The tickling fetish is out there, known, common to the both of us, waiting to be experienced, anticipated in general, but not necessarily at this very moment.
It'd start with a tease. a mention. a word. She'd then look at me, teasingly exposing an inviting ticklish, vulnerable spot: Maybe raising an arm above her head, exposing an armpit, an exposed waist picking from a short blouse. Maybe sliding a foot towards me on the sofa, or punting it in my lap. it would start slow, it will increase to a point when holding her down is mandatory. Maybe, she would even say "you are going to have to hold me down for that..."