Micro.blog Help has instructions for setting up Custom domains:
Point your custom domain to Micro.blog.
If you host your microblog with a paid Micro.blog plan, consider pointing a custom domain to it. This could be something like microblog.yourdomain.com or even just yourdomain.com. Mapping a custom domain to Micro.blog is included free, as long as you already have your domain registered.
Read the instructions: Custom domains Point your custom domain to Micro.blog. Published December 8, 2015.