ER service performed at outside hospital

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We are a psych facility and have a client who was sent out to the emergency room at a local hospital. I am told that per contract, we pay the hospital directly for the ER services at 20% of the total cost. I'm also told that I need to add a revenue code to our UB for the ER services (we are billing Medicare). Does anyone know what revenue code I should use? I've checked my UB editor and it seems that 0450 may be what I want to use -or maybe 0900? Please provide any information that you may have.