API feeds is to pull data from Micro.blog - for example replies. To enable cross posting from your existing blog, a setting can be enabled. From the help page on getting started [1]
> "If you already have a blog, such as one based on WordPress, connect it to Micro.blog by adding the feed URL to your account under Account → “Edit Feeds”."
A specific example is given on the IFTTT [2] page, but I am unsure if that is the best way to achieve RSS cross postings on micro.blog.
[1]: https://help.micro.blog/2017/api-feeds/
[2]: https://help.micro.blog/2018/ifttt/