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.
You can only input playing techniques that exist, ie that are included with Dorico by default or that you have added in the Edit Playing Techniques dialog. You should find that once you’ve created custom playing techniques, they’re available both in the popover (accessible using the popover entry you specify) and in the corresponding section of the panel, and you can input custom playing techniques into the music just like factory default ones.
If you don’t want to create custom playing techniques but instead add text more freely, you can use text objects with either manual text formatting or using a particular paragraph style.
At the bottom of each Playing Techniques category (in the right panel of Write mode) there’s a + button and a pencil button. The + button will always open the Playing Techniques Editor and create a new Playing Technique. If you select a Playing Technique on the page - the score or part layout in front of you - then the pencil icon with open the Playing Techniques Editor to edit that Playing Technique. If you haven’t selected a Playing Technique on the page, the pencil button will indeed add a new Playing Technique.