To form chords (two or more notes with different pitch at the same time) you can add more note-heads to one stem:
position the cursor horizontally at note that should receive one more head.
position the cursor vertically to desired position (cursor-style changes)
Notes inserted this way are displayed gray on the working area to distinguish them from the main-note. Secondary notes are not taken into account when determining bar line. On the status line they can be recognized having an ampersand (&)