On CO, you can enter your MP# which will be recognized for access to the elite zone and free ELR from booking (on CO metal) as well as free E+ including exit row/bulkhead from booking (on UA metal). If you prefer using your OP#, you can always switch to your OP# once the linked status goes through.

You can also book on UA, though I hate their website so much, I would only do that if their price is cheaper.