There are a number of ways of doing it, but they all depend on what routing possibilities your mixer has.
1 set your recording path to go out of a subgroup (say outputs 1 & 2)
2 use your channel fx to go to an outboard reverb unit
3 bring the fx return into another subgroup (say outputs 3 & 4)
4 you should be able to listen to your original as well as the reverb, but only the dry original will be recorded.
Alternatively (if you have headphone amp with an extra line in):
1 use an aux or monitor send to go the reverb unit
2 bring the reverb return into the headphone amp aux in
The trick is to find a way to listen to two paths, where the first is the dry path going to the PC, and the second is the wetpath that goes nowhere except to what you are listening through