I've been wondering recently how people sing or scream into the pickups of their guitar, and I was wondering how do you do that? Like what amp settings work best when doing that?
pick up your guitar and put the strings against your throat and you can sing through it
Not sure why you'd want to do that, especially when the microphone was invented before the guitar pickup. But anyway...

Yeah, you need to vibrate the strings, since their vibration is what changes the magnetic field and creates the small electrical signal. As already mentioned, placing the strings against your neck, or shouting into the pickup area/soundhole would also work.

Amp settings? Loud. Really. Loud.