Is there an API for iOS's Assistive Touch Feature?

Does iOS SDK provide framework or API for us to use their Custom Gesture creation?

How do I verify Apple Pay with assistive touch?

You can use AssistiveTouch to perform a double click on your Apple Watch side button, such as when you confirm a payment using Apple Pay or unlock your Mac. Open the Settings app on your Apple Watch. Tap Accessibility > AssistiveTouch. Tap Confirm with AssistiveTouch, then tap Continue.

There is no public API for either accessing the existing Assistive Touch feature (it has to be turned on by the user in the settings) or for creating a similar UI.