This extension require a callback that is not yet present (at RT 4.0.0 time) in share/html/Ticket/Display.html.

To add it, got to /path/to/your/rt, then run the following command:

For RT 4.0.0:
patch -p1 < /path/to/this/extension/rt-4.0.0-AfterMessageBox.patch

For RT 3.8.10:
patch -p1 < /path/to/this/extension/rt-3.8.10-AfterMessageBox.patch

