How to edit fretboard diagram palette

I'm familiar with how to edit a diagram once it's placed in the score, but I can't seem to find any options for editing or adding templates in the palette.

I've skimmed https://musescore.org/en/handbook/3/fretboard-diagrams, https://musescore.org/en/node/289836, and https://musescore.org/en/handbook/3/palettes#custom-palettes, none of which seem to give specifics of how to do this, just that it's a common thing to do (unless perhaps it's supposed to be exactly the same as, say, custom time signatures, in which case the relevant options are missing!). There are no additional diagrams that can be added to the palette with the "More" dialogue, and no option I can see for saving an edited diagram for future use. What am I doing wrong?