var xhr = new XMLHttpRequest();
var url = _spPageContextInfo.webAbsoluteUrl;
if (url === "undefined") {
    console.log("_spPageContextInfo.webAbsoluteUrl undefined");
    url = "http://siteUrl";
}

xhr.open('POST', url + "/_api/web/Lists(guid'bec2b6d5-3183-4bd7-87a5-0989610b2e25')/Fields(guid'341eeecb-e5ff-447c-9cd0-84c9f00c80c6')/setShowInEditForm(true)");
xhr.setRequestHeader("Accept", "application/json; odata=verbose");
xhr.setRequestHeader("X-RequestDigest", document.getElementById("__REQUESTDIGEST").value);
xhr.setRequestHeader("X-HTTP-Method", "MERGE");
xhr.setRequestHeader("If-Match", "*");

xhr.onload = function () {
    if (xhr.status === 200) {
        var kk = JSON.parse(xhr.responseText);
        console.dir(kk);
    }
    else {
        console.dir(xhr);
        alert('Request failed.  Returned status of ' + xhr.status);
    }
};
xhr.send();