How do I register a paper URL into doai.io or oadoi.org?
doai.io relies on BASE (Bielefeld Academic Search Engine)'s data. So to  register a paper into doai.io, one has to put it in an open-access repository that is harvested by BASE. There are more than 5700+ indexed repositories, the complete list can be found here

oaDOI.org relies on its own harvesting of open-access repositories, that are listed at https://api.oadoi.org/data/sources.csv