At one point I downloaded Chambers Thesaurus from the App Store and somehow–I don’t know how-- it managed to place itself within the context menu when right-clicking text in Scrivener. I have no idea how I did that, but it was wonderful and I do not seem to be able to reproduce that behavior in Scrivener 3. Any ideas?
There are methods by which software can insert contextual menu items into other programs (in general, so you probably had that in Mail, Safari, etc. as well as Scrivener), but as to which method they were using precisely I couldn’t say—they would know better.
Some common things to check though:
System Preferences: Extensions
System Preferences: Keyboard: Shortcuts: Services
That, for example, is where Nisus Thesaurus provides global access. You right-click on a word and from Services choose “Look Up Selection in Thesaurus”.