API assistance request

I create users for our Society website using the CSV upload facilities, This includes registering the user on PMS. The heading line for the CSV file is this: Username,Email,first_name,last_name,display_name,password,role,subscription_plan_id

I have now discovered the WordPress REST API and so I can create users without having to log in to the website. Unfortunately I cannot seem to be able to include the PMS registration like I did with the CSV version. Is there a way to include the subscription plan in with the json part of my POST request?