I know it hasn’t been easy to find out the JSON notation for some of the document entities in Vidispine. Good news! Since 4.2.2, there is a hidden call for getting this format.

The resource to be used is PUT /APInoauth/debug/echo.

An example using cURL: 
curl localhost:8080/APInoauth/debug/echo -Hcontent-type:application/xml -Haccept:application/json -d@- -X PUT << EOF
my other value


which will give the result:

{"timespan":[{"field":[{"name":"field_a","value":[{"value":"my other value","mode":"add"}]}],"start":"-INF","end":"+INF"}]}


 I know what you are thinking. 

 "Can I trust that that resource will stay?" Yes, indeed. 

 "Are there other hidden resources under the debug path?" No, not yet :-)