Using the lookup dialogs in customised web app

Goce

Im creating a customised web app that creates workspaces. My problem is that i want to use the worksite lookup dialogs for (users, groups, custom2 and custom3) by creating a url to GetLookup.aspx and passing my parameters. I can get the lookup window to display for custom 2 and 3 with the correct populated information but i cant get the selected value back into my form. I get a permission denied error.

Can someone please tell me if what im trying to do can be done or if i would have to make my own lookup windows?