use the preamp with the microphone for vocals and plug it's output into one input of the Y-cable Gary posted above.
plug the guitar directly into the other one - use a mono channel for each on the mixer (well, what else... )
if your guitar has active circuitry (it needs a battery) then it's a perfect connection.
otherwise (usually Strat or Tele, passive circuitry), there's an impedance mismatch, which will steal some tone from the guitar pickup.
But no harm will be done to anything connected.
It depends on personal taste how much you tolerate the signal loss. You may compare it to a clean channel on your amp.
Possibly the latter has a lineout ? You could use that as well to plug it to the Y-cable.
cheers, Tom