There is no notation for the thumb tap that is commonly agreed upon. It can be notated by a symbol directly above the tab that would be explained either next to the symbol or at the end of the sheet music.

The thumb tap uses the same motion of a thumb slap, but instead of hitting the string, the either the side of the thumb or the thumb nail hits the guitar's soundboard.

To do a thumb tap with the side of the thumb to get a mellow bonk sound, straighten out your thumb to avoid hitting the guitar's soundboard with the thumbnail.

To do a thumb tap with the thumb nail to get a sharp click sound, bend your thumb to hit the guitar's soundboard with the nail.

To get a combination of both a bonk and a click sound, hit the guitar's soundboard with a mix of the side of your thumb and the thumb nail.