By default, the only programmable button on the Voice Messenger, which is the lower key on the left side of the device, is set to Voice Commander. 

You can reprogram this button to open up a different application by selecting Start > Settings > More > More > Button Assignment.  Then you can scroll through the options there.  Unfortunately, you are are limited to the following applications:

• Voice Commander
• Windows Live
• Windows Media Player
• Pictures & Videos
• Tasks
• Contacts
• Calendar
• Internet Explorer
• Messaging
• Messenger

I don’t use Voice Commander much and so I wanted to use this button to open up a different application.  I searched around the Voice Messenger’s registry and figured out how to assign the button to any application that is installed on the device through a simple registry edit.

Most all applications on a Windows Mobile device are opened through the use of shortcuts found in the \Windows\Start Menu folder (there are also some subfolders in this folder, like games, that include applications as well), so I used Total Commander (my preferred file explorer application) to look through the applications there to find the correct shortcut for the application I wanted to use (in this example I am assigning the key to the Camera) and wrote down the entire path for this shortcut (\Windows\Start Menu\Camera.lnk).

To program the left key to always open up the Camera, I used Total Commander’s registry editor (SKTools Standard also includes a registry editor that could have used for this) to navigate to:

HKLM > ControlPanel > RedefineKey > LowerLeftSide > Open

Then I changed the reference there to \Windows\Start Menu\Camera.lnk, and selected OK and exited all the way out of the registry.