Synchronous call from JavaScript to Dynamics 365/ Dataverse Web API using XML HTTP

function validateEnrollment(ec)
{
var formContext = ec.getFormContext();
var studentid = formContext.getAttribute("modern_student").getValue()[0].id;
studentid = studentid.replace("{","");
studentid = studentid.replace("}","");
var courseid = formContext.getAttribute("modern_course").getValue()[0].id;
courseid = courseid.replace("{","");
courseid = courseid.replace("}","");

var fetchXml = "<fetch version='1.0' mapping='logical' no-lock='false' distinct='true'><entity name='modern_enrollment'><attribute name='modern_enrollmentid'/><attribute name='modern_name'/><attribute name='createdon'/><order attribute='modern_name' descending='false'/><filter type='and'><condition attribute='modern_student' operator='eq' value='stid'  uitype='modern_student'/><condition attribute='modern_course' operator='eq' value='csid'  uitype='modern_course'/></filter></entity></fetch>";
fetchXml = fetchXml.replace("stid",studentid);
fetchXml = fetchXml.replace("csid",courseid);

var encodedFetchXML = encodeURI(fetchXml);

var query = "/api/data/v9.2/modern_enrollments?fetchXml="+encodedFetchXML;

var globalContext = Xrm.Utility.getGlobalContext();

var finalpathwithquery = globalContext.getClientUrl() + query;

var data = null;
var isAsync = false;

var req = null;
if (window.XMLHttpRequest) {
    req = new XMLHttpRequest();
} 
else if (window.ActiveXObject) {
    req = new ActiveXObject("MSXML2.XMLHTTP.3.0");
}

req.open("GET",finalpathwithquery,isAsync);
req.setRequestHeader("OData-MaxVersion", "4.0");
req.setRequestHeader("OData-Version", "4.0");
req.setRequestHeader("Accept", "application/json");
req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
req.setRequestHeader("Prefer", "odata.include-annotations=\"*\"");
req.onreadystatechange = function () {
    if (this.readyState === 4) {
        req.onreadystatechange = null;
        if (this.status === 200) {
            var result = JSON.parse(this.response);
            data = result;
        } 
        else {
            Xrm.Utility.alertDialog(this.statusText);
        }
    }
};
req.send();

if(data.value.length >0)
{
      Xrm.Utility.alertDialog("There is already an enrollment exist for the same student with same course.");
      var saveEvent = ec.getEventArgs();
      saveEvent.preventDefault();
}
}