Subform Data in v2 REST API

What is the mechanism for adding subform data in the Creator v2 REST APIs? There is nothing documented in the Data APIs documentation (https://www.zoho.com/creator/help/api/v2/).

I was able to determine how to GET the subform data by adding it to the report and specifying the fields required along with a delimiter, which is wonky, but works (the data is in JSON - why not just add an element in the JSON for each of the subform fields instead of appending them all to one element?)

However, I cannot find anything for how to add or update subform data for a form. This had been done using the XML RPC APIs in v1, but that is going away as well, and does not support OAuth, so we need to migrate away from that as well.

Can we add the records directly to the subform? But how will they be linked to the main form? Can we add multiple records in one call, liek we can with XML RPC? I don't want to have to make 3 REST calls to update one form and its 2 subforms.