I can't use POST /_xpack/security/user/_search {} as that thinks I'm trying to add a new user.
What is the API to search users?
There is no API for searching.
You can retrieve all users with GET /_xpack/security/user
or retrieve users by username GET /_xpack/security/user/jacknich,rdinero
Does that mean I can't search user metadata?
I'm reading this and trying to test the search for metadata before applying a role query template
Does that mean I can't search user metadata?
It means you can't search for users with specific metadata.
I don't follow why you're trying to do that in the context of the ABAC post.
Just trying to see if I get the search query correct before adding it to the template. I've had lots of weird issues with syntax for the role template in the past.