Can OON Dr Prescribe RX in HMO?

If a client buys an HMO, but primary care doctor is NOT in network, and she is willing to pay cash to that primary OON doctor (to keep her doctor), can that doctor prescribe Rx and have it covered under Rx benefits of the plan?

Or, in order for Rx to be covered, it has to be from an IN network PCP???

Content of insurance-forums.net