To be able to add notes by their positions right on the staff you need first to switch to the Staff Positions input type in the toolbar or by using .
Thus the cursor starts to indicate the current staff position (line or space) to which it's possible to add or remove notes. To navigate between staff positions you can make use of a mouse or keys.
Accidentals (Staff Positions) palette is purposed for toggling natural, sharp, flat, double sharp, and double flat notes in the selected position. It is available in the Actions sidebar with the Accidentals button.
Another option to toggle notes in the current position is pressing appropriate shortcut keys.