Hi,
If you are trying to set the value for Existing Account field then you can use the below code (generated from rest builder)-
================
var entity = {};
entity.subject = "test";
entity.lastname = "test last";
entity["parentaccountid@odata.bind"] = "/accounts(F6E4610A-8D9B-E811-811E-E0071B685A91)";
var req = new XMLHttpRequest();
req.open("POST", Xrm.Page.context.getClientUrl() + "/api/data/v8.2/leads", true);
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.onreadystatechange = function() {
if (this.readyState === 4) {
req.onreadystatechange = null;
if (this.status === 204) {
var uri = this.getResponseHeader("OData-EntityId");
var regExp = /\(([^)]+)\)/;
var matches = regExp.exec(uri);
var newEntityId = matches[1];
} else {
Xrm.Utility.alertDialog(this.statusText);
}
}
};
req.send(JSON.stringify(entity));
==================
Hope this helps.