Advise on using Expression on HSDL Reference Attributes

Summary:

Looking to pull in a Reference Attribute and use it to return the Last Name of the Worker on an Maintain Assignment template for reporting through HCM Integrations Real Time. Does anyone have any experience with EL in HSDL or thoughts on how to accomplish this?

Content (required):

Each time I try to define the field using an expression I am getting an error. I assume this is because I do not have the Person Name object being returned anywhere in this template. I tried adding a VO for Person Name on the Reference Attribute and selecting the Person Number and returning the Last Name along with it, but this is not working for me. I've already used one Reference Attribute to pull in the Person Number which was very simple to set up based upon an existing attribute and is very helpful in