I'm looking at various sources of API documentation, including these:

• https://bd-api.ncsa.illinois.edu/docs/
• PDF brochure

None of these describe the JSON requests that are expected at the end point in question. It is hard to imagine a normal, lazy developer inferring the expected JSON. They would have to look at the code, as I did. I have the answer I needed, not a block, but I'm putting in this ticket for future API users.