Hi, 

let's inspect the issue you're facing and eliminate all the possibilities.

On your Firebase project settings, make sure you have "Firebase Cloud Messaging API (V1)" enabled.

On your Google Cloud console (click on "Manage Service Accounts") make sure you have generated a service account key.

If you don't, create one using JSON option, once generated use this JSON file on your calls for the Firebase authenticator extension in the ServiceAccountJSON parameter as a binary. You can upload it as a resource on your module.

Once all that is out of the way, now you can use the GetToken action.

If you want to get a token to use it on the messaging API, send this value on the Scopes:
"https://www.googleapis.com/auth/firebase.messaging"

Then on the following calls for any endpoint under https://fcm.googleapis.com pass it on the "Authorization" header with the "Bearer " prefix like this:

That should be enough to get the token and authorize your API calls, you can try on Postman with your generated token. 

If you're planning to use it on Service Studio, make sure you replace the URL (/v1/projects/palpites-da-loteria-2/messages:send) on the OnBeforeRequest callback.

The OutSystems server performs an encoding on the URL, replacing the ":" with a "%3A". 

So create the OnBeforeRequest callback and just create an assignment like this:

CustomizedRequest.URLPath = Replace(Request.URLPath, "%3A", ":")

Otherwise you will get weird runtime errors.