When sending messages through WebSockets, the "Payload" text area does not make use of the CodeMirror JSON plugin and instead uses the single line CodeMirror variant just like the Payload title input field and the request/connection URL input field. The lack of features like auto close braces or quotes, indentation, etc. while typing makes it hard to use this part.

There is a "Beautify JSON" button but it's inconvenient to use.