I am using AJAX for the first time and try to send data to my PHP controller with an XMLHttpRequest. Unfortunately, the data arrives empty there.

           request.onload = () => {
                let responseObject = null;

                try {
                    responseObject = JSON.parse(request.responseText);
                } catch (e) {
                    console.error('Could not parse JSON!');
                }

                if (responseObject) {
                    handleResponse(responseObject);
                }
            };

            const requestData = `title=${this._title.value}&reference=${this._reference.value}&area=${this._area.value}`;

            request.open('post', 'checknews');
            request.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
            request.send(requestData);

Whats wrong ?