If am correct the only way to input a playing technique that is not included in the panel is via the popover.
The Engrave → Playing Technique window is not for direct input into the score, as far as I know.
What I experienced is that I wanted to add spiccato and clicked on the “+” on the panel but this action adds a new music symbol, whereas I want to browse the available techniques.
When using the mouse instead of the popover one may add an already existing technique just because it does not show in the panel.
As for the spiccato I don’t remember whether I added it or if it was already included in Dorico, but the strange thing is that in “Play mode” even though it is labelled as “Natural” it is played spiccato by NotePerformer.