When you select a page its ID is appended to the QueryString, but you should better get the selected page from the PageEditor as shown below
Guid pageID = ((Telerik.Cms.Web.UI.PageEditor)(
The properties get enabled when you create a page, so the correct place to add your properties is under PagePropertiesEdit.ascx
Another option is subscribing for CmsManager.Executing and check the command name
sender, Telerik.ExecutingEventArgs e)
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