Although recording MIDI is a common way to create new MIDI regions, it is often desirable to do so as part of editing/arranging.

To create a new MIDI region with the mouse, simply enter draw mode (press "d" or click on the pencil tool) and then Left-click in a MIDI track. This creates a region that is one bar long. Alternatively, use the left button to drag-create a new region of the desired length.

Once the region exists, to trim it to a different length, switch back to grabber/object/select mode (press "g" or click on the grabber tool). Then trim to your desired length.

Once the region is ready, add some notes to it.