Add Custom Filter for Lookup in Dynamics CRM

function onPageLoad()
{
    var LookupControl = Xrm.Page.getControl("new_marketinglist");
    if (LookupControl != null) {
        var fetchXml = "";
        LookupControl.addPreSearch(function () {
            LookupControl.addCustomFilter(fetchXml);
        });
    }
}