You can input clefs using the clefs and octave lines popover, both during note input and by adding them to existing music. You can also use the popover to change the type of existing clefs.
Many instruments in Dorico Pro have variants that show alternative clefs according to the layout transposition by default. You can select the appropriate instrument variant from the instrument picker when adding or changing instruments.
Procedure
Result
During note input, clefs are input at the caret position on all staves across which the caret extends.
Clefs added to existing music are input on each selected staff, at the rhythmic position of the earliest selected either note or item, according to your item input position setting. When you add clefs to a range of selected items, restorative clefs are also added at the end of the selection.
Clefs apply to all notes on the staff until the next clef or the end of the flow, whichever comes first.