The way I know to bind any external auth with Rancher is as admin go into the config for users or authentication sources (they were near each other in the top left menu) and with the versions I used (been a while) you could set exactly one external source.
After doing that you could log in with a user and then go back in as admin and set the privileges you want for the user. After that you can log in as the user and download your own kubeconfig for the user.
There are potentially more scriptable ways, but that's what I was aware of and I don't have a Rancher instance to poke at just now.