Can't Connect to 'Endless Golden Rail' Tutorial

Hi folks. My son has been working on the ‘Endless Golden Rail’ Tutorial. As per the instructions, we’ve loaded the Windows 10 version of minecraft, started a new world, and entered ‘/connect m.tynker.co/jtworld10

The game responds by establishing a connection to the server, then immediately disconnects and crashes minecraft. This is incredibly frustrating! Any ideas what we can do?