How can I authenticate with google cloud in Unity? - unity3d

Question

I'm trying to use google automl object detection in a Unity project.
Here is the google curl command example:
curl -X POST \
-H "Authorization: Bearer $(gcloud auth application-default print-access-token)" \
-H "Content-Type: application/json" \
https://automl.googleapis.com/v1beta1/projects/${PROJECT_ID}/locations/us-central1/models/model-id:predict \
-d '{
"payload" : {
"image": {
"imageBytes" : "/9j/4AAQSkZJRgABAQAAAQ … "
},
}
}'
After some googling, I start to create a UnityWebRequest to test response:
WWWForm form = new WWWForm();
var postUrl = "https://automl.googleapis.com/v1beta1/projects/1043345783500/locations/us-central1/models/IOD1798528344157847552:predict";
using (UnityWebRequest www = UnityWebRequest.Post(postUrl, form))
{
www.SetRequestHeader("Authorization", #"Bearer $(gcloud auth application-default print-access-token)");
yield return www.SendWebRequest();
if (www.isNetworkError)
{
Debug.Log(www.error);
}
else if (www.isDone)
{
Debug.Log(www.downloadHandler.text);
}
}
So the response from google is:
{
"error": {
"code": 401,
"message": "Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.",
"status": "UNAUTHENTICATED"
}
}
I understand that, I am not unauthenticated but couldn't understand how I can authenticate. Do I have to make an authentication request? How? Do I have to post something to get an access token? Then where should I send it?
I downloaded some credentials from google server but I really don't have any idea how to use it...