Adding values to SubForm through Script

Adding values to SubForm through Script

New to Creator: 

I have a single line field in a subform called added by.  I would like to be able to articulate who added the new entry.  

My code is simply
input.addedby = zoho.loginuser;

However, this doesn't work in the subform.  Only in the main form.  Is this somehow not supported? 

Dale