how do I http request a google app engine service? (datastore admin)

[Jackery Xu via StackOverflow]

I would like to backup to datastore using a http request on /_ah/datastore_admin. I am getting 403 error when I try posting:

https://ah-builtin-python-bundle-dot-myapp.appspot.com/_ah/datastore_admin/backup.create?key=mykey&name=BackupToCloud&kind=user&filesystem=gs&gs_bucket_name=mybucket

Am I doing it incorrectly or is this just impossible with what google is currently offering?

Please DO NOT REPLY directly to this email but go to StackOverflow:
http://stackoverflow.com/questions/44011234/how-do-i-http-request-a-google-app-engine-service-datastore-admin