I forgot my AirPlay pass code. How can I get it?

You cannot forget an AirPlay passcode. It's auto generated randomly, and displayed on the TV every time you need to connect to the AppleTv

If you forgot the AirPlay password, then you'll need to go into the Apple TV settings, and change it. You don't need to remember it to change it.

Instructions here:

Use Conference Room Display mode and related settings on your Apple TV - Apple Support