Welcome to the Make community!
According to
- php - Sending email with signature using Gmail API - Stack Overflow, and
- How to include the users Gmail signature when using GmailApp.sendEmail from Google Sheet - Stack Overflow
It is not possible to automatically append your signature to emails drafted/sent via the API.
You will need to call the Gmail API endpoint “users.settings.sendAs.get” to fetch your signature(s) and append the one you want (if there is more than one signature).
However, Make doesn’t have a module for this particular endpoint (Get Gmail Signatures), nor the Gmail integration have a universal module (Make an API Call).
If the external service has a Developer API Reference/Documentation then you should be able to integrate the endpoints in Make using the generic HTTP “Make a request” module.
You can also suggest for it to be made in the Idea exchange. Don’t forget to search for it first, just in case someone already suggested it, so that you don’t end up creating a duplicate.
If you need assistance in setting up the generic HTTP OAuth 2.0 module, please provide additional information about what you have tried with regards to the external service’s Developer API Reference – how you are setting the connection up, a link to the endpoint are you trying to connect to, and what errors you are encountering.