Populate email fields by using email template

            var body = "";

            var subject = "";

            var titleInfo = "title info";

            var req = new XMLHttpRequest();

            req.open("GET", parent.Xrm.Page.context.getClientUrl() + "/api/data/v8.2/templates?$select=body,subject,templateid,templatetypecode,title&$filter=startswith(title,'" + titleInfo + "')", false);

            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);

                        for (var i = 0; i < result.value.length; i++) {

                            body = result.value[i]["body"];

                            subject = result.value[i]["subject"];

                        }

                    } else {

                        Xrm.Utility.alertDialog(this.statusText);

                    }

                }

            };

            req.send();

            /*Parsing Subject */

            if (DOMParser) {

                parser = new DOMParser();

                xmlDoc = parser.parseFromString(subject, "text/xml");

            } else {

                xmlDoc = new ActiveXObject("Microsoft.XMLDOM");

                xmlDoc.async = false;

                xmlDoc.loadXML(subject);

            }

            var _subject = getInnerText(xmlDoc.childNodes[0].childNodes[1].firstChild.nodeValue);

            /*Parsing Body */

            if (DOMParser) {

                parser = new DOMParser();

                xmlDoc = parser.parseFromString(body, "text/xml");

            } else {

                xmlDoc = new ActiveXObject("Microsoft.XMLDOM");

                xmlDoc.async = false;

                xmlDoc.loadXML(body);

            }

            var _body = xmlDoc.childNodes[0].childNodes[1].firstChild.nodeValue;

            if (_subject != "")

                Xrm.Page.getAttribute('subject').setValue(_subject);

            if (_body != "")

                Xrm.Page.getAttribute('description').setValue(_body);

//

function getInnerText(html) {

    var tmp = document.createElement("DIV");

    tmp.innerHTML = html;

    return tmp.textContent || tmp.innerText || "";

}