 Flirting with an autistic person can often achieve the exact opposite of what you want to happen. I've been in many situations where I thought someone was just being mean or nasty to me, so I pan them off and leave the interaction with a negative view of them. But later, my friends would tell me that it was flirting. It's important to be explicit and direct when you like an autistic person. It might feel strange or weird, but showing clear signs of attraction, interest, and kindness is likely your best shot.