You’ll need to lengthen the actual MIDI Part. Either by dragging the Part End in the Project window, or if you switch to the Key Editor, you can drag the Part Boundaries (so long as you have enabled the “Show Part Borders” icon in the Key Editor’s toolbar).
In fact there is a way to extend the Part, from within the Score Editor… just record some MIDI while the Score Editor is open. (You can then delete the extra recorded notes, if you wish).