Are you using the Simple Entry Tool or the Speedy Entry Tool?

With the Simple Entry Tool:

While the note is selected, go to

Simple menu > Simple Edit Commands > Add Interval > Unison

Depending on what shortcut table is selected, there may even be a keyboard shortcut for adding a Unison.

With the Speedy Entry Tool:

Position the crosshair on the note, but not on the note’s scale step.

You can e. g. position the crosshair a second above.

Speedy menu > Speedy Edit Commands > Add Note

There is even a keyboard shortcut for adding a note.

When the note is added a second above the first note, drag the note down to a Unison.