Need guidance on capturing web page post string...

Question

I'm looking into writing a security tool to test web pages that send post data upon form submit.  In a basic .Net WebBrowser control, when the user submits a form, I want my program to capture the post string before it is sent, display it to the user where they can add in a pre-made hack string (sql injection, java alter scripts ect..) and submit the newly edited post string to check against their web site for vulnerabilities 
What I can't seem to find is information on is how to capcher that post string.  I know it can be done because there is a tool out there called TamperIE that does what I want my program to do but TamperIE is a IE add-on and I'm looking to make a stand-alone winform.
Any tips, helpful links, direction or guidance would be great.
Thanks,
-J