As in any other simulator: add an AC component to you supply voltage; tie you inputs to signal ground by properly setting the DC voltage.
Perform Monte Carlo AC simulation and look at you output node.
If your system is an amplifier, then instantiate your main cell and make it to work in differential mode by using a copy of the AC source you use to determine the PSR. Then divide your differential gain by yout PSR
PSRR= Ad/PSR
If your system is, say, a bandgap, then PSRR=1/PSR