ansible_callback_service in Sat6

I'm using ansible_callback_service to Tower and I'm trying to pass extra_vars to the playbook but it's not picking up the extra_vars. Below is the curl command from the ansible_callback_service snippet, is something setup wrong?
Thanks,

curl -k -H 'Content-Type: application/json' -XPOST -d '{"host_config_key": "st-u-ff", "extra_vars": "{"ansible_playbook": "playbook.yml"}"}' https://server.localdomain:443/api/v2/job_templates/1/callback/