You can test sending messages using the FCM REST API by sending a request through cURL.

curl --header "Authorization: key=<API_KEY>" \
       --header Content-Type:"application/json" \
       https://fcm.googleapis.com/fcm/send \
       -d "{\"registration_ids\":[\"ABC\"]}"

_{Syntax retrieved from here.}

The API_KEY indicated above is referring to the Server Key that can be seen in your Firebase Console's Cloud Messaging Tab.

The part where:

"{\"registration_ids\":[\"ABC\"]}"

is, can be replaced with your own payload. See the FCM HTTP Protocol Documentation for more details.

To do this in Postman, you simply have to set the following:

1. Set request type to POST
2. In the Headers, set the following:
   • Content-Type = application/json
   • Authorization = < Your FCM Server Key > (See your Firebase Console's Cloud Messaging Tab)
3. Set the payload parameters in the Body (in this example, we used the raw option, see screenshot (2))
4. Send the request to https://fcm.googleapis.com/fcm/send

Screenshots:

(1)

Note: Always keep your Server Key a secret. Only a portion of my key is visible here so it should be fine.

(2)

(3)

Notice that the request was a success with the message_id in the response.