Create a therapeutic alliance with them:
Get on their level and join in on whatever they’re doing
Treat them as if they were neurotypical, say hello, talk to them, ask them questions, give them options/ free choice, etc.
Be creative with play:
Grab blocks and an action figure and have the action figure save someone at the top of the tower
Playdough! Mold it to spell their name or numbers. Count the numbers
Stick something in their pocket and giggle while pointing at it. Make your pocket bigger so they can put something in it