Sorry, I got confused between @ioannis explaining how to get values from Firebase and your question about how to save values to Firebase. To save a value, you would do something like this:
set cloud variable usuarios/NffX8NEIaLhtYqm4lg9vnpllXHo1/Compras/Codigo to “222222222” or you can set the whole object at /usuarios/NffX8NEIaLhtYqm4lg9vnpllXHo1 with many different values by creating an object to replace the one that’s there.
You can see an example here: How to save text inputs to Firebase? - #2 by jared