<wctags:userPicker id="userPickerId"
pickerCallback="myPickerCallback" />

function myPickerCallback(objects, pickerID)
{
var updateHiddenField = document.getElementById(pickerID);
var updateDisplayField = document.getElementById(pickerID + "$label$");
var myJSONObjects = objects.pickedObject;
for(var i=0; i< myJSONObjects.length;i++) {
var oid = myJSONObjects[i].oid;
// Here “fullName” is the displayAttribute requested
var displayAttr = eval("myJSONObjects[i].fullName");
updateHiddenField.value=oid;
updateDisplayField.value=displayAttr;
}
}