Something like this - gets the phone number from a parent account:
function GetValue() { var parent = Xrm.Page.getAttribute("parentcustomerid").getValue(); if (parent != null) { //Perform any basic checking to make sure we have a valid starting value if (parent[0].entityType == "account") { var accountId = parent[0].id; var serverUrl = Xrm.Page.context.getClientUrl(); //If pre UR12 use getServerUrl() var oDataSelect = serverUrl + "/XRMServices/2011/OrganizationData.svc/AccountSet(guid'" + accountId + "')?$select=Telephone1"; var request = new XMLHttpRequest(); request.open("GET", oDataSelect, false); //false = synchronous, true = asynchronous request.setRequestHeader("Accept", "application/json"); request.setRequestHeader("Content-Type", "application/json;charset=utf-8"); request.onreadystatechange = function () { if (request.readyState == 4) { if (request.status == 200) { var account = JSON.parse(request.responseText).d; Xrm.Page.getAttribute("new_parentaccountphone").setValue(account.Telephone1); } } }; request.send(); } } };