You need to have an instance of Root in your microflow before you can pass it into the Export Mapping. So you can simply do a Create Object action for Root at some point before the Call REST action. This should now be available in the Parameter dropdown.
I hope this helps.
hi Robert ,
i have create the instance of Root in microflow but it gives error . what is the neef to create the obj of the Root because it is empty donot have attributes .