identify portal user

identify portal user

When a user is logged on and makes a change to a field I want to record that fact. I am using the code...

if (input.Live_Status  =  "Visit Confirmed")
{
    input.Confirmed_By = input.Added_User + " at " + zoho.currenttime;
}

but it is not returning the portal user it is returning the main account name.

What code do I use to identify the portal logged in user.

Thanks