appending permalinks

I need to append my permalinks in the following way so as to pre-populate my Gravity Form field:
http://fluidframe.com.au/wp/portfolio/test-item/?imageid=photoname

However, when the permalink resolves it gets changed to:
http://fluidframe.com.au/wp/portfolio/test-item/%3Fimageid%3Dphotoname

Which obviously does not work. How do I stop it from changing the URL

Am also using an plugin called “custom permalinks”