Using qmmm options in json format

How do I use qmmm extern options provided by psi4 in json format. qcschema does not provide any such option.
For example in psi4 input format I would use:
Chrgfield = QMMM()
Chrgfield.extern.addCharge(Q,X,Y,Z)
psi4.set_global_option_python(‘EXTERN’, Chrgfield.extern)

How do I execute same thing in json format.