Posting data from a form using Javascript / AJaX to a Tomcat jsp page returns null field values [duplicate]

Using Javascript to build a form with POST. When the form is submitted using AJaX to a Tomcat jsp page (or a servlet), the jsp page isn't retrieving form data.request.getParameter() is instead returning null.Also getting the same result when attempting to POST the form data using 'await fetch(test.jsp)'

A test javascript function to POST data to a Tomcat jsp page:

function send() {    xhr = new XMLHttpRequest();    const formData = new FormData();    formData.append("recId", "5");    xhr.addEventListener('load', handleReturn);    xhr.open('POST', 'test.jsp', true);    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");    xhr.send(formData);    // xhr.send("recId=5");    return xhr;}

But in the target script 'test.jsp' -String rId = request.getParameter("recId");is returning null. Expecting '5'

This is the payload reported by Dev Tools:------WebKitFormBoundarysRv6NCasQdzagUnsContent-Disposition: form-data; name: "recId"5------WebKitFormBoundarysRv6NCasQdzagUns-

Have also tried using a form with a field named recId, eg:formData = new FormData(this-is-a-form);xhr.send(formData);

Same result.String rId = request.getParameter("recId");returns null instead of '5'

Have tried all the hints I could find online, and the documentation I found suggests this should work.What am I missing?

However, this (not using a form) does work:

function send() {    xhr = new XMLHttpRequest();    // const formData = new FormData();    // formData.append("recId", "5");    xhr.addEventListener('load', handleReturn);    xhr.open('POST', 'test.jsp', true);    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");    // xhr.send(formData);    xhr.send("recId=5");    return xhr;}

This is the payload reported by Dev Tools:recId=5