New Post: set property from dialog

I created a project from WinSharp Managed Setup - Custom Dialog template

On custom dialog user should choose a parent web site in combo box.
Also I have a property IISWEBSITE_NAME. How to set this property to selected item from dialog?

(I tried pass session into class constructor and set set property via session in SelectedIndexChanged event, but as a result combo box disappears from form at all )