Which widget are you using? Exactly that part in your printscreen is blocked by your widget pop-up from the Modeler...
I guess you are using the "Charts" widget. https://appstore.home.mendix.com/link/app/105695/
In the documentation of this widget, I found the following:
REST URL - The URL used to retrieve data. Only used when the datasource is REST endpoint.
For more details: REST endpoint "
Which part are you having problems with configuring?