Here's one way that works to make you pee in the bed.
When I was younger my brother did this too me. He took a bowl of warm water and placed my hand in the bowl of warm water one night when I was 6 years old and I wet my bed.
When I found out he did that to me I did that same thing to him the next night and I got revenge because he wet the bed too.
So maybe place your own hand in a bowl of warm water while you drift off to sleep or have someone place your hand in the bowl of warm water when you're asleep and as long as you have some fluid in your bladder you're sure to wake up in a wet bed.