Update an existing form to add a new field

Hi All,

New to SM, 

I need to update an existing form in SM to add a new field to contain a url.

Confused with the input of the existing text fields of the form. Is it just a variable or is this any function/script invoke . Can anyone please help how to add a new field to the existing form.

For example .I want to add a new text field name link , can I give the input as $RD.Attach.link ,after i have created the field in database records.