FYI I did workaround this by first uploading an XML schema VLT'ing the form back to my project and amending the content.xml for the xml schema to be application json then replacing the XML with a JSON schema. But would prefer to do it through the UI
DarrenBiz is absolutely correct. All you got to do is save the file with extension as schema.json and it would let you create the form with JSON schema. You may keep the schema on the repository or just upload it via disk, either would work. I used the same sample schema given in the document and here is the result.