You can input MIDI trigger regions using the MIDI trigger regions popover.
Prerequisite
You have chosen the appropriate middle C numbering convention setting.
Procedure
Result
During note input, MIDI trigger regions are input spanning the duration of the selected note or item, which is usually the last input note.
When adding MIDI trigger regions to existing music, they span the selected duration on each selected staff. If you selected a duration within an existing MIDI trigger region, the new MIDI trigger region replaces the existing region only for the selected duration.