Seems like a really basic question, but I couldn't work this out.

I have a node with two relays. I just want to be able to turn either one on or off by a rule. I can toggle them on or off via a widget, or from the action board, but I can't work out how to do it from an operation.
Could someone please point out the steps of doing this. I assumed it was go to operation -> add operation -> send payload -> sensor variable (select relay sensor status) and send payload. But I couldn't get this to work.

What am I missing?