Populate subform from REST

Have an existing subform on the form and would like to create the process and initially populate the subform with multiple rows of data using the REST CreateProcInst. I can populate the first row of the subform, but nothing after.

Not sure how to create additional subform rows from REST.   

Any thoughts?

https://documentation.agilepoint.com/11/developer/restmethodCreateProcInst.html