Close your eyes and imagine peeing your pants/sitting on toilet and peeing/simply peeing/ rushing water/rushing river after drinking so much water that you feel like you are going to explode. This worked for me so well I just peed my pants!
How To Force Yourself To Pee?
Think of something that makes you laugh LOTS. You will laugh so much that you will pee.
Try drinking LOTS of water or tea or something