I've got a need to populate a UserForm with values if a user clicks through to the form with a particular route. For example a contact us form (with name, email and comments), if you go directly to it then the form is empty and you need to fill everything. However if you go via the page "I'm interested in Silverstripe" and click the "contact us and find out more" then the form field "comments" is pre-populated with "I'm interested in Silverstripe".
Can anyone give me some pointers for this?
I've tried setting the session with the form data, but this hasn't worked yet and if it did it doesn't account for changing the form.