To add a rear chase bar to your 2015 RZR XP4 1000 you need to order this part:
https://www.utvwiring.com/2015-2018-RZR-Dual-Circuit-Taillight-Power-Adapter_p_18.html
The Taillight Power Adapters come in two configurations:
CC1 - only has an output for a License Plate Light or Whip Light
CC2 - has an output for a License Plate Light and also and output for a Rear Light Bar that give you Tail, Brake and Ground output wires.