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

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

The motion can be more awkward than the thumb slap strum as it requires you to open your palm more and gives you less control over what strings you're hitting.

Just like the thumb tap, the sound of the tap can change depending on if you use your thumb nail, the side of your thumb, or a combination of both.