@feditips got it working yesterday morning but I needed to install his WebFinger plugin too, and if it's a multisite, you need to activate both plugins on each blog.

The docs for configuring it aren't very good unfortunately.

Being a blogger, of course I wrote a blog post about it, which you can find at my blog's Mastodon address of @donncha@odd.blog or on the boring old web at https://odd.blog/2022/11/06/how-to-add-your-blog-to-mastodon/ :)