I successfully created a node that calls a 3rd service API, but I want to add a credential feature to that node. The thing is that I’m not getting how to properly grab the credential value to make a Basic auth.
Did you have a look at the documentation here? There it is described how it should work. It looks like you are currently mixing together two different things which should be in separate files (credential definition + node definition). I could write here what is exactly wrong and how it should look like but it would take quite a while to write and I would then just repeat what is already documented under the above URL.