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.
Changing the sound of the thumb tap
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.