Go to the bathroom and “get lost.” You’re in a strange place. It’s plausible. Slip out a side door and leave.
They mean well, but these things can be such bullshit. These days, I just thank them for the invitation and send my regrets. If they want to make an issue of it, they can be my guest. The fact is, they need my autism (to show diversity and disability in the workforce) as much as I need their job, so neither of us is going to make a fuss.