You can input playing techniques with the keyboard by using the playing techniques popover, and with the mouse by using the Playing Techniques panel. Pedal lines are considered playing techniques in Dorico Pro because both affect the sound that the instrument produces.
You can input string indicators outside the staff in the same ways, using either the playing techniques popover or Playing Techniques panel. However, you can only input harp pedal diagrams using the playing techniques popover.
You can input string indicators inside the staff using a property in the String Indicators group of the Properties panel.