Ton, I haven’t looked into it directly myself, but based on this article from 2015, it would seem that it was being done via xmlrpc at that time. If I recall correctly, individual platforms are responsible for building an maintaining their integrations on IFTTT now, so it may be done via WordPress API now, particularly since the API is much more mature now compared to 2015. Possibly worth digging into or asking IFTTT or WordPress directly.