via QUERYSTRING
You do not need a unique Web Form for each individual source. You can override the Webform Action by adding &source=your+source to the end of your "Get Link" URL.
For example, if you want to set the source to "Google Adwords" for specific visitors, use the following URL
http://www.domain.com/Form.aspx?Key=youruniqueformkey
&Source=Google+Adwords
Important: 1) Replace spaces in your URL with a plus sign "+". 2) The exact source (without the plus signs) must exist in your Source library.
via FORM ACTION
You can use the same method for the "Get HTML" option. Change the "<form action..." line in the same manner:
<form method="post" action="https://www.domain.com/FormHTML.aspx?
Source=Google+Adwords">
via a DROP DOWN
You can override the source by creating a dropdown (SELECT) field in your HTML form. Set the "name" attribute to "source", for example:
<select name="source"><option value="Google Adwords">Google Adwords</option></select>