Thanks for further explanation on the process - not having done OAuth2 before I wasn't sure what to expect where or when. With more time spent on the host issue, this led to PROGRESS! 🙂 The suggestion on the website for a redirectUri is "https://localhost", and what I've found - in the end (cos at first I changed the code to match the website, not the other way around!) - was that the program ONLY works if you include the port and "authorize" (so "https://localhost" won't actually work, even if you had https://localhost in the code). I suggest you change the hint text on the website to include a port number and "authorize" (even if someone goes and changes them themselves. Situation not helped by freqent mentions of "you can put whatever you want there" - no you can't. If you don't include a port number it doesn't work. Ditto for the "authorize" if it's in your code. The only optional bits are http or https:, localhost or 127.0.0.1 - they all work either way. At least I've solved that now!).
So, went through the authorisation, and now the SimpleTest app is getting stuck a bit further on, at var context = await http.GetContextAsync();. Not crashing, just hung, therefore no error messages at all. Not sure what I need to do to get this line working? (I've let it sit there for several minutes now and still no sign of a response)
Note, the app was opening up a tab on my already-open browser. Just in case this was an issue, I closed my browser and reran the program (so it had a new browser all to itself), but it still hung. I can see the token there in the URL (yay! 😉 ), but it's not making it's way past that. 😞
Also, you're right. I was runing SimpleTest,but when I clicked on it to check the host I had accidentally clicked on the one underneath! Oops.