Doesn’t seem to be a way to do this? Entering sfp in the pop-up results in fp sub.
The best suggestion (for the moment) has been to enter sf and then move the caret a sixteenth or thirty-second note forward on the grid and then enter p.
In the Dynamics Properties Panel change the Subito forte style.
(Edit) for the whole Project, change the “Appearance of subito for forte” in the Engraving options for Dynamics.