JSON API Calls to Message Center in Campaign v8

Is it possible to send JSON format API calls to the Message Center in Adobe Campaign Classic v8?

In this documentation for an older version of Campaign Standard, JSON format API calls to the Transactional Message Center are possible: Documentation .

However, in Adobe Campaign v8, I have not found any documentation indicating that JSON format API calls to the Message Center are possible - only XML. I have tried sending JSON format calls with the proper 'content-type' header and received an "invalid document structure" error.