So I found another URL: https://marketplace.zoom.us/docs/sdk/native-sdks/windows/c-sharp-wrapper which explained how to copy binaries and .h files (why?), but more importantly set the run mode to BUILD.
This now shows the start_join_meeting dialogue.
Given the adoption by Zoom of mandatory passwords, there is no password field for joining a meeting, so I added one to the XAML and C#:
What is the “Access Token”, and UserID/UserToken required to start a meeting?