So, assuming you actually want to carry on with this monkey business....
If you are not getting an erection in the first place, viagra or equivalent won't help. They help strengthen/ maintain an erection. That is not your issue, which much be psychological.
I would strongly recommend finding a SP you can discuss the problem with in advance. I'd suggest booking an hour, with the clear intention that you will NOT be having penetrative sex. Probably not even oral on you- just a sensual experience with nothing more than hand-relief, if you get hard.
But do not go into it with any expectations of an orgasm. Just view is as a sensual tactice experience. Enjoy that experience. If you DO get to orgasm, then it is a pure bonus.
It is trite, but true, that it is really about expectation management.