@rmdes In your case, it should be firstname.lastname@example.org. I'm not sure how picky it is about formatting, but here's a direct link to mine as an example. Notice how it isn't all on one line like yours. Micro.blog removed the line breaks when I posted the steps.
If you wanted to be verified at your root domain instead, you could follow similar steps with whoever is hosting that. But it should work through Micro.blog with the subdomain.