How to hook digital input to remote control buttons?
Usually the keys are multiplexed so you cannot easily use an output directly. 

You could, however, emulate the key closures with an analog switch such as a 74HC4066 quad analog switch. The ground on the remote should be commoned with the controlling circuit. Power for the 4066 should be the same as the MCU I/O power. 

You would send a pulse to the analog switch to emulate the key press- maybe 250ms.