Adding Content-Type to a Net.HTTP.request()

I have a simple function that sends some JSON to my API.

However, when the server receives it, it says the the Content Type is invalid as it is '' (empty).

var saveAssessment = app.trustedFunction(function() {
    const params = {

       cVerb: "POST",
       cURL: "http://localhost:8089/api/step-by-step-assessment",
       oBody: assessmentJsonString,
       aHeaders: {
          "Content-Type": "application/json",

       },
       oHandler: {
          response: function(msg, uri, error) {
             if (error != undefined) {
                app.alert("Error ocurred: " + JSON.stringify(error));
             } else app.alert("Success!");
          },

       }

    };

    Net.HTTP.request(params);
});