Autofill Lookup Field OnLoad

Autofill Lookup Field OnLoad

Hi, 

I'm trying to set a lookup field (Form B) to autofill with the most recent value from a string field in Form A. 

I can get the lookup to autofill OnLoad when I link it to a system field (e.g. ID), however, it doesn't appear to do the same when using a user-defined field. 

For example, the script:
"input.Lookup_Field = Form_A[Added_User = zoho.loginuser].ID sort by Added_Time desc;"
works perfectly, but 
"input.Lookup_Field = Form_A[Added_User = zoho.loginuser].String_Field sort by Added_Time desc;"
does not. 

Can anyone tell me why and/or offer a solution?

Thanks in advance!