How Do you play a celebrity?

I often see streamers such as kingscrusher doing live streams but I have never been able to find where to play them - is it just random chance to encounter them or is there a special waiting list to play them when they are online?

Challenge them

Depends on streamer. Some of them just play in random tournaments, some say they are accepting challenges from viewers, some of them just play randomly in pool and it so goes on.

Oh ok, so you need to catch the start of their show to know where to get at them?

Yes.

monthly tournaments or you pay them mwhahah.

I played GM Yasser Seirawan accidentally in a tournament one time, he was (obviously, but still surprising) really strong.

Add Eric Rosen,
When he streams on twich, he accepts challenges at that moment.