HTTP GET submits parameters in the URL and does not have a body like POST. For example...
https://onesignal.com/api/v1/notifications?id=1&msg=hello
Parameter can also exist as part of the URL. In Web API these are called route parameters and match a configured pattern.
https://onesignal.com/api/v1/notifications/1/hello
You'll need to read the API documentation to understand how to format the URL.