From https://kubernetes.io/docs/reference/access-authn-authz/authentication/#users-in-kubernetes
All Kubernetes clusters have two categories of users: service accounts
managed by Kubernetes, and normal users.
Kubernetes does not have objects which represent
normal user accounts. Normal users cannot be added to a cluster
through an API call.
Even though a normal user cannot be added via an API call, any user
that presents a valid certificate signed by the cluster's certificate
authority (CA) is considered authenticated.
So there is no API call to create normal user. However you can create service accounts that can have RoleBindings bound to them.
Another possibility is to create TLS certificate, sign it with Kubernetes cluster CA (using CSRs) and use it as a "normal user".