On each controller, there is a small button that allows you to turn it on and off but this button also allows you to synchronize with your box.
In Recalbox, in your available system lists, press Start on your usual controller and go to CONTROLLER SETTINGS.
In this menu, select PAIR A BLUETOOTH CONTROLLER.
Once the search for Bluetooth devices is launched, press the sync button of the controller to associate. It should start to have the 4 lights next to it going back and forth from left to right and vice versa.
Once the search for Bluetooth devices is complete, you should see your controller in the list.
Select the controller and validate. Your controller should now be recognized and still have the 4 lights going back and forth.
Once the association is complete, the controller will appear in the list of controllers.
Now you can enjoy playing with your SNES controller!
Once associated, the controller will have an automatic configuration detected with the HOTKEY button on SELECT.