Making changes to the Message Edit page

Hi,

I need to integrate Outlook OWA 2013 with some email encryption software. In previous versions I have just edition the basiceditmessage.aspx and the editmessage.aspx files to get it working, however in the new version this doesnt seem to be the case. All I need to do is to add a drop down form when users is creating/editing a message & then some javascript will run. Any ideas on where I start with the new version of OWA?

Thanks