How can I use REST API authentication in Mendix?

I have designed an API REST service (with Bonita) to which I can perfectly connect with Postman, with the following parameters:

By the way, the x-www-form-urlencoded option that is selected comes from the Content-type application/x-www-form-urlencoded header that is not displayed in my screenshot. The official Bonita specification states that this header is needed and I always get a 200-OK status code as an answer.

How can I specify an equivalent request with the body part in a Mendix Call REST service in a microflow? Here is what I have so far:

I guess the body part should be specified in the Request tab, but I just don't know how to do it properly. I always get the following error message for my connector, which means that, whatever I specify, the username is not taken into account:

An error has occurred while handling the request. [User 'Anonymous_69a378ed-bb56-4183-ae71-c9ead783db1f' with session id '5fefb6ad-XXXX-XXXX-XXXX-XXXXXXXXb34f' and roles 'Administrator']

I finally found that the proxy setting was the actual problem. It was set at the project scope and simply clicking on No proxy in the General tab did the trick! (both services are hosted on my local machine so far)

I just had to fill in the dedicated Authentication field in the HTTP Headers tab then, with the correct credentials, to eventually log in my Bonita service.