Paypal api – How to send emails with Gmail REST API

I need to send emails from my server, through simple REST calls, from my backend-server email address, to the email addresses registered in the system.

But there is a problem, bear with me.

The thing is a lot of the documentation I’ve seen so far seems to assume I want to access the clients gmail data, which I don’t, I only use the client’s address as a destinatary.

A lot of the examples I’ve seen, involve a visual promt to authorize the access to the api.

BUT WHY?

They assume I’m gonna be making apicalls from a mobile device, acting on behalf of the client, which needs a visual prompt for consent. But none of that applies to my use case.

The thing is, there is ever only gonna be one sender, "ME" (the company email designated for the backend server).

I should be able to create a secret api_key on the google console, and send that in my request headers (like freaking FIREBASE does), or use that key to generate a token which I then send to the api endpoint (like Paypal does).

I want to be able to do something like:

POST https://gmail.googleapis.com/upload/gmail/v1/users/{userId}/messages/send

Authorization: key=<key_created_on_google_console_or_token_obtained_using_said_key>

{
  message stuff...
}

The api reference on google says that I need to create an authorization using OAuth credentials, which I’m then supposed to use to create a short lived token that is inserted as

"Authorization: Bearer <TOKEN>"

in my next apicalls, until it expires. But…

In the console, creating a new OAuth2.0 client ID, says I need to create an app. But I already have a proyect and an api_key (with permissions to gmail api). And that said app is gonna have to be sent for review!. What’s going on?.

So..

How do I send emails, from my own-controlled email address, using REST calls.

It’s all server side, no need to access any user data (not even my own), I already control the sender address, I already have created a project on google console, I already created an api_key.

OAuth seems to think I’m doing something I’m not, so what am I missing?.

Thanks.