This operation is only available while a MIDI song is played and the player is currently paused. Use the MIDI Player Status dialog to move current play position to a location where you want to add a marker (e.g. intro, refrain, verse, scale transposition ...). Use the pause operation to hold this position and then start the insert marker operation.
A dialog opens where you can enter a text that describes the mark (single line). Then use OK button to write the marker into the current active MIDI song (be sure that the correct MIDI file is active if more files are open).
The new marker will be known to the player only when you play the new result MIDI file.
Abort the dialog using cancel if no modifications are wanted.