Not sure if you can do it mate.
Quote by death_metaller
**** burzum XD

there i said it...kinda

edit: i do realise they are not here...so i kinda didnt say it...

I don't think you can really do feedback in powertab, but you can use 'artificial harmonics' (put the octave to '15ma') to create an annoying high sound that resembles it (a bit, it's still MIDI).
In the beginning, the Universe was created. This has made a lot of people very angry and is widely considered a bad move.