Help with ?= variables

Here is my page:

http://www.domain.com/index.php/contact/

I want to trigger a thank you message when a user signs up to a newsletter on the same page using an ?f= variable like so:

http://www.domain.com/index.php/contact/?f=yes

I want the than you message to come up if the URL has an f-value. How come wordpress can’t recognize these?

http://www.domain.com/index.php/contact/?f=yes
http://www.domain.com/index.php/contact?f=yes

Any ideas on how I can make this work?