I've always put it here for Oklahoma, and never had an issue. I don't have REP for a CO return at the moment, but that's where I would start. You're likely to get an Oklahoma letter requesting proof of the withholding, so keep a copy of the K-1 and any other OK withholding documents, even if you also send them in with the 511-EF.

I know this is a year old, but I had the same question and found out how, so wanted to share in case others search in the future.

Withholding from Line 16 of CO K-1 goes to Form DR0104CR Line 11. 

Enter it on Screen 53.261 Colorado credits under Colorado Refundable Credits line "Tax paid by composite return or SALT Parity election"

Be sure to attach the CO K-1 to return.