Solution
To send a message by trigger while scheduled messages are being sent, refer to Example 06 - Transmit Scheduled and Dynamic Messages located at: C:\Program Files\NI\LVAddons\ballardinstr32\1.6.0.3\examples\Astronics Ballard Technology\BTI429Lib\Example 06 - Transmit Scheduled and Dynamic Messages

You need to install Astronics Ballard Avionics Driver to use the sample. To find/select your hardware, you need to use BTI Editor and LabVIEW as described in here.