How to fix the PF at the POI?
How to fix the PF at the slack bus?
From your question I unsderstand you should be modelling a generation plant and trying to maintain reactive power/ power factor at POI.
You can set fixed Qgen for the generator (keep same value for both Qmax, Qmin) and with iterative method you can adjust this acheive required reactive power corresponding to that powerfactor.
Or other way would be you can adjust Voltage of the generator to control the reactive power flow at POI.
The slack bus generator will always inject the necessary Mvar to keep the scheduled voltage. Adjust the Mvar injection by nearby generators, fixed shunts, switched shunts, FACTS, etc., in order to change Qgen of the slack bus.