Dude, you are probably a bit old.
An undercut is just short on the sides. That's it. Nobody said pompadour or slick back.
The last time I was at the barber shop, a man probably fifty got an undercut. He had a birds nest. The way it was cut, it blended his balding in with the fade. It looked good. He actually looked middle age. A hood barber is great.
Another look is doing a undercut, shirt faded or skin faded sides, and fringe forward or just downward if someone has a huge forehead and receded hair line.
A good barber, bit of beard, and or stubble looks good. There comes a time where you just bite the bullet and buzz or Shaving is your alternative.
I suspect that you are much older since you automatically assumed that undercut is a forehead haircut lol also, I get your hair is Norwood? Likely more than mine do, this isn't something that you can do. It's ok.