How do I add applications to the Messaging Menu?

Applications that are in the messaging menu each have a file in the system directory of:

/usr/share/indicators/messages/applications/

That directory contains files that have the paths to the desktop files for those applications. You can also have one of these in your home directory with this path:

~/.config/indicators/messages/applications/

So let's say that I want to add Thunderbird to my messaging menu. I'd do something like this:

$ mkdir -p ~/.config/indicators/messages/applications/
$ echo /usr/share/applications/thunderbird.desktop > ~/.config/indicators/messages/applications/thunderbird

You'll need to restart your session (log out and back in) the first time you create the applications directory.

Here's an example of usage from a python application.

Documentation and example code for doing this can be found here on the Developer Portal.

_{Editor's note: page no longer exists, so I have replaced the dead link with an archive snapshot from June 2013. Other links on the linked page may be broken and information out of date}