In Metadata pane, perhaps should add “+” and “-” (and corresponding functionality) to Custom Meta-Data sub-pane “…”, similar to as done for Keywords subpane.
Appears that in Scrivener 3 for Mac OS X, the gear (options?) icon is used rather than “…” and is the only button available, so presumably should disregard this.
Yes, there’s not a way to add new metadata fields directly from the inspector, so this needs just the button to open the project settings to the custom metadata where you can set that up.