Hi Jurjen,
Using the Uniface JavaScript API you don't need to directly perform HTTP GET/POST requests from/to a URL in a REST style - you can just use the uniface API's activate() method to directly invoke Uniface operations with parameters (they will be asynchronous, so you may want to use JavaScript promises to wait for a result .then do something).
In this kind of interface, I often find it useful to convert JavaScript objects to JSON and back with standard JavaScript commands, e.g. JSON.stringify() and JSON.parse(), and use Uniface's struct commands (e.g. structToJSON, jsonToStruct, structToComponent, and componentToStruct to convert data on the server).
If you want to use a REST approach (e.g. continuing to use the Javascript Axios API) that is also possible, but it's a different topic - in summary you would need to create a Uniface REST component to provide an interface.
Regards,
David