I am having a similar problem with populating custom fields on registration.
The standard setup uses the firstname and lastname to populate the 'name' field in the users table, and works fine

I have set up 2 new fields FirstName and LastName in the DB, and 2 new Key fields in Postman to match these, but can't work out how to get Postman to populate these. Am I missing something?

Ok. My site changes were through modified users.xml etc. so normal registration process works fine via front end and admin.
So although the jBackend post registration works for me, you are saying it will only work correctly with a plugin?

I fully understand your solution using Profiles plugin, but that's not what I have done.

Let me rephrase that if that makes it easier.
I have edited the users table directly, so the new fields are in that table. I then modify the xml files so the registration process works seamlessly as if it was core Joomla.

My question is very simple: Can jBackend populate fields directly to ANY field in the users table, i.e. does it only recognise Joomla core fields or it doesn't care, it will also populate my new custom fields too.
If YES, then I should be able to do this, but my payload does not work for some reason (User gets registered BUT new field are just ignored)
If NO, then maybe I need to consider a custom solution/mod to jBackend

I hope that is a better explanation