What is the best way to make a content negotiation request for a DOI from any Registration Agency?

Content negotiation is possible for all DOI Registration Agencies (e.g. DataCite and Crossref) via doi.org. Following the guidance outlined in the documentation, you can make a content negotiated request like this:

curl -LH "Accept: application/vnd.citationstyles.csl+json;q=1.0" https://doi.org/10.1021/acs.joc.0c00770

DataCite's content negotiation service is available at https://data.datacite.org and only supports DataCite DOIs.