There are currently the following ways to query the database from other software. If you need to query data that is not covered by the API, just drop me a line.
A query to https://www.qslinfo.de/api/v1/call/{callsign} returns the information for the queried callsign in JSON format.
DXspider can query the database by the command sh/db0mdx if configured properly.
A HTTP POST request needs to be sent to https://www.qslinfo.de/qslinfo.asmx, containing
at least <callsign>QUERY_CALL<callsign>. It will return a reply in XML format.
CLX queries the database via the following URL (callsign passed as GET parameter): https://www.qslinfo.de/directsearch/directsearch.aspx?call=8Q7ZZ