Hi Zack Frankowski,
From the error you’re seeing: "BadRequest: Invalid FcmV1 credentials", this usually means Azure is not able to validate the service account credentials due to one of the following common reasons:
Even though the Firebase Cloud Messaging API (V1) is enabled, Azure expects the full, unmodified service account JSON file in a specific format and any deviation (like extracting fields manually or malformed line breaks) will cause this error.
-  Generate the correct service account key:
- Go to Google Cloud Console – Service Accounts.
 - Choose your Firebase project.
 - Click “Create Service Account” (or select an existing one).
 - Go to Keys => Add Key => Create new key (JSON).
 -  This will download a 
.jsonfile with your credentials. 
 - Paste credentials into Azure: In the Azure portal, go to your Notification Hub, Navigate to: Settings => Google (GCM/FCM) Remove any legacy FCM server key if one is present.
 
- Choose the "Firebase Cloud Messaging (V1)" option.
 -  From your downloaded 
.json: Paste theproject_id,client_email, and fullprivate_keyexactly as-is (including\nline breaks). Save the configuration. 
- Make sure the service account has the Firebase Admin SDK role or Editor/Owner permissions under IAM.
 
Note:
- Do not edit or reformat the JSON.
 - Ensure Firebase Cloud Messaging API (V1) is enabled in Google Cloud Console – APIs & Services.
 - Avoid pasting individual fields, Azure needs the full-service account format.
 
https://free.blessedness.top/en-us/azure/notification-hubs/configure-google-firebase-cloud-messaging
Hope this helps, if you have any further concerns or queries, please feel free to reach out to us.