Activate a light over a RFD900 mavlink connection?

Hello there,
I got 1 pair RFD900 telemetry. I will use one of them on a RC drone, other one will be on the ground next to me. Can I send specific datas by using RFD900s? For example I will plug a light bulb on RC drone, can I send ‘turn on the light bulb’ command if I make necessary connections and programming when the drone is in the air? If so could you give tip or short guide me how to do that?

Yes, it is possible.

Read the documentation about relay outputs to configure and connect the FC side.
Read the Mavlink documentation to understand what you need to do on the GCS side.