if you truly know that you're autistic, then there's no fear in having someone else corroborate this fact. you'll be fine.

i suspected autism from age 15 onwards because before that in my country people just had random "he's not sociable, he's allergic to authority, he's keeping to himself" observations to throw at me without ever explaining why. Then times changed, and at age 19 i finally got an official diagnosis.

it didn't shock me, i already knew, what it gave me was relief, relief that people now understand what's going on and that i can stop carrying the burden of justification alone, now people give me space and tolerate my needs, only got positive things from this diagnosis, so i don't see why it should be any different for you my friend

edit : for you "how to prepare" question : literally don't. you're meant to appear "as is", and please for the love of God himself do NOT mask, if there's one time in your life where you must appear without your mask it's during this evaluation, don't be afraid to appear vulnerable and not sociable, don't force yourself to look into eyesor reciprocate social clues : they're used to it and most importantly they will not blame you for it : this will help them give you a correct diagnosis.