To get someone to wet the bed while sleeping you supposed to put their hand in warm water. Also can try making a leaky faucet sound, which tends to trigger people to have to go to the bathroom.
The best way by far to make sure you wet the bed, is to have someone you trust, tie your hands to the bed before you fall asleep. You will actually end up sleeping very deeply in this manner (speaking from experience).
The body then goes into a form of 'hibernation mode' where the bladder is capable of storing a lot of liquid before it 'needs to go.'
But, when you wake up, be forewarned that if you are not untied within minutes of waking up, you will wet the bed badly, because the minute your body realises that it is awake, it then lets your bladder know that it wants to have its load released - and in a hurry.
A way to guarantee that you wet the bed as well, is to make sure you have 2 glasses of water approximately 30 to 45 minutes before falling asleep, because by then, your bladder is already starting to send the "I need to pee" message to your brain. So when you wake up, and you realise you're tied up, it will be a matter of 10 minutes or less, and you will wet the bed - guaranteed.