Cannot seem to get this to work.

Fields:

Services (multi select)

Service_Date (date)

Service_Date_List (text)

When the user selects [Services] and a [Service_Date], the text "Service Service_Date" gets copied to [Service_Date_List] 

When the form is submitted, I want to append the info in [Service_Date_List] to the field [COMMENTS] as a log.

The following does not work in (on Validate) or (on Success)

 if (input.Service_Date  !=  null)
{
    insert into Tracker
    [
        COMMENTS = input.Service_Date_List
        Added_User = zoho.loginuser
    ]
}