Gmail apps script: Create an add-on that adds a button to gmail that can automatically forward the email to a predefined address

I want a button in gmail that when you select an email, you can click a button and it will automatically forward the email to “[email protected]” or so. The email address with be assigned in the script. How would I go about making this?

The Google App Script is a bit confusing to me and I’m not sure how to test any scripts I have tried.