How to use batch REST API with custom User ProfileFields

I'm using the batch.json REST API to create 50 users at a time, which works well.

E.g. I'm sending _REQUEST_X_DATA like:

Username%3Daaaaa3%26Password%3Dpasswordaaaa3%26PrivateEmail%3Daaaa3%40localhost.com

I would like to also set some custom profile fields I have created myself, like FirstName. With the normal users.json API, it means adding a ProfileFields tag like:

But how do I set this custom field within the _REQUEST_X_DATA parameter value used for the batch.json API?

Thanks in advance, appreciated.