How do I add a shortcut for AssistiveTouch to my iPhone's Control Center?

To add an AssistiveTouch shortcut to your control center go to your iPhone Settings > Control Center 

Tap on Accessibility Shortcuts

Then, go to Settings >Accessibility > Accessibility Shortcuts > click AssistiveTouch.

To turn it on and off quickly open your iPhone's control center, click on the Accessibility icon, and click AssistiveTouch.