Dorico Pro automatically selects the register of pitches during note input, but you can override this and select the register manually.
During note input, Dorico Pro automatically selects the note whose register is the smallest interval away from the previously input note. For example, if you input an F and then press A, an A is input a third above the F, rather than a sixth below.
You can override this automatic register selection in the following ways:
Register selection when inputting chords
During chord input, Dorico Pro automatically inputs notes above the highest note at the caret position. For example, if you press A then E then A, a chord of A-E-A is input at the caret position.
You can input notes below the lowest note at the caret position instead by pressing Ctrl-Alt (Windows) or Ctrl (macOS) as well as the letter for the note name; for example, Ctrl-Alt-A (Windows) or Ctrl-A (macOS).