Manually add an RFXTRX switch (nexa)

(newbie warning)
I’m sitting with a couple of Nexa switches (PER-1300), the ones that you stick in a wall socket and then you can adda a ligt or anything you want to turn on or off.
If I have a remote it would be really easy to add them, but I have no remote.

I understand that I need to add configuration in the configuration.yaml and I can’t for the life of me google an aswer on how to do it.
I found a way to create an ID under developer tools → template but now I’m dead in the water.

Can someone give me advice or point me to where to find information?