I'm trying to add my Google calendar to Thunderbird without success

I have followed the help article describing how to add my Google calendar to Thunderbird, but the process stalls at the first hurdle. I have synced my calendar in Google, but when I get to the screen asking for my username and the location, I can't find what to put in the location. I have tried the basic Google address, and I have tried the Google calendar address https://calendar.google.com/calendar/ - but whatever I try, Google comes up with "400. That's an error. The server cannot process the request because it is malformed. It should not be retried. That's all we know." I'm hoping that someone on the forum will know rather more, please.