I know there is a free text when you can paste your JSON parameters & values but they can’t be fed from the application. It would be good to give the choice to specify how the parameters should be sent (URL vs Body).
As to when you use body or parameters … it tends to depend on the complexity of the date being passed in, simple parameters (i.e. the majority of APIs) … I tend to use the parameter. However in some cases you are better off sending stuff in the body … for example some maths in mathjs.