Problem using Mailgun integration

I am trying to use the send email action which comes with Mailgun but I keep getting an error “Forbidden” and I can’t seem to resolve the issue as there is no documentation or clear instructions on what’s happening. Here is a video:

Was anyone able to use this integration that could highlight what data did you populate the action with?


Anyone tried this integration?

Is it possible all this time no one needed to integrate with Mailgun as email sender?
I am still having problems using this integration…