You can input harp pedal diagrams using the playing techniques popover, both during note input and by adding them to existing music.
If you do not input any harp pedaling, Dorico Pro assumes all harp pedals are in their natural setting, as they would be for C major. Any pitches that do not fit with the current harp pedaling, excluding the two lowest harp strings, appear red when colors for notes out of range are shown.
Procedure
Result
The corresponding harp pedal diagram is input at the selected rhythmic position. Depending on your per-layout settings, it is either displayed as a diagram, using note names, or not shown and instead indicated by a signpost.
During note input, harp pedal diagrams are input at the caret position.