Please have a look at this which should explain how the simple json plugin works: Grafana simplejson dashboard
Seems to me you’re trying to use https://api.opsgenie.com/v2/alerts as the url when configuring simple json datasource. That don’t work. To use the simple json datasource you probably need to have a service in between which receives request from simple json datasource, do a request to https://api.opsgenie.com/v2/alerts and then translate and return a response that simple json datasource understands. The service in between is where you’ll put the logic for authenticating with opsgenie, like custom http headers.