“Simple” lets you choose which fields you want to add in the webhook request body (aka payload).
“Advanced” lets you write your own payload data structure, using template variables (placeholders). You can see available variables by clicking “Insert placeholder” on the top right of the text box.
I couldn’t find any references on whether “Content type” in this screen refers to the HTTP (Update: the “Encoding” field on this screen sets the request
Content-Type header. My guess is it isn’t since the
Content-Type header in the webhook request is anyway set to
Content-Type header. The “Content type” field by contrast affects the request body. )
I guess Advanced is useful if your webhook receiver endpoint (callback URL) needs a specific object structure. But the UI is confusing.
You can also click the “Test your URL with current settings” to trigger a request to the “Callback URL”. See screenshot below: