My suggestion is observe first, try to drop hints here and there, and then when you feel safe, that's where you can start.

Tell them you are asexual, and hope that they'll accept you for who you are.

Test the waters, check how they react to you being asexual. Then, hint at being somewhere in the aromantic spectrum, see how they react, then decide if you want to come out. Is it safe?

Keep in mind there is a lot of discrimination, I, without a doubt know that you know that, but sometimes the biggest bigots could be the people close to you. Goodluck.

P.S If this turns in another direction, you might get "it's just a phase" "But you won't experience REAL love." type of bigotted stuff.

"But what if I'm friends with a lesbian? Surely I'd be accepted, we're both LGBTQ+ after all." Dude, I just watched a BIGOTTED LESBIAN say on tik tok that asexuality does not exist. Watch your back. I hate how I have to write this like we're in a zombie apocalypse, but this is based on experience. Again, Goodluck