Radio button selection based on link data

I have Drupal 6.x, I can't be more specific.

I have a form with fields Department (select), Message (textarea), and Email (hidden).

In the Department field (Parameters), the following options are set:

email1|Department 1

email2|Department 2

email3|Department 3

email4|Department 4

a link of this type to the form /content/66

So, I need to add SOMETHING to the link so that depending on this SOMETHING, one of the radio buttons is selected when this page is opened. How can this be done? Or perhaps another way to ask the question is how to use PHP to select the desired radio button?