How do I sign a string with my PGP private key?? Tearing my hair out can't figure it out!

Alright I'll make this fast. Sorry to bother y'all, but I've run up against a brick wall and I'm not sure how to go any further. I've been using the DNMs for years, but this is the first time I've encountered something like this.

One of the sites I am trying to sign up for asks me to "sign the following string with your PGP private key", and then gives an encrypted (I think) string of numbers/letters/etc. I'm usually pretty savvy with things like this but I've tried every possible way I could think of and did a hell of a lot of googling and got squat, so here I am.

Anyone know how to do this? I use GPG4win/Kleopatra as my PGP program on my (windows) computer. Thanks in advance!