Add a hidden text field and place one of the two following merge tags into it. (I'm not sure which one you need):
This should capture where the visitor came from (in the sense that it will tracker the website on which the link was placed). I'm not sure if you can do logic with a hidden field, but if you can, then you can use logic based on the value in the hidden field to control the appearance of additional fields based on the referrer value.
You'll have to so some experimenting, but hopefully this was helpful.