[emath]

I've done it successfully with XHR, If someone else will need to do this, here is an example code :

                var xhr = new XMLHttpRequest();
                var formObjData = {
                    timezoneoffset: 2,
                    dst: 2,
                    'options[adminemail]': 1,
                    'options[showemail]': 1,
                    do: 'addmember',
                    password: $("input[name='password']")[0].value,
                    passwordconfirm: $("input[name='passwordconfirm']")[0].value,
                    username: $("input[name='username']")[0].value,
                    agree: $("#agree")[0].checked,
                    email: $("input[name='email']")[0].value,
                    emailconfirm: $("input[name='email']")[0].value,
                    securitytoken: 'guest'
                };
                var strObj = '';
                
                for (var k in formObjData) {
                    if (formObjData.hasOwnProperty(k)) {
                        var val = formObjData[k];
                        strObj = strObj + `${k}=${val}&`;
                    }
                }
                strObj = strObj.slice(0, strObj.length - 1);
                xhr.open('POST', './register.php?do=addmember', true);
                xhr.onreadystatechange = function () {
                    if (xhr.readyState == 4 && xhr.status == 200) {
                        // sucess code
                    } else {
                        if (xhr.status !== 200) {
                            alert('error);
                        }
                    }
                }
                xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
                xhr.send(strObj);