From this article on page 673:
The "von Kármán line" appears to be what mathematicians refer to as a "folk theorem", arising out of a conference discussion but never formally published by him. It was fleshed out in later publications, especially in the influential work of Haley (1963) and there is some justification for calling it the "von Kármán-Haley line".
Question: What was that conference and when and where was it held ?