Welcome to the Atlassian developer community, @JuanJosGarcaMerino!
No, you cannot create users with
POST /rest/api/3/user. As the documentation indicates, this resources is “experimental” with the following caveats:
This resource is retained for legacy compatibility. As soon as a more suitable alternative is available this resource will be deprecated.
Apps cannot access this REST resource.
That includes Forge and Connect Apps.
Right now, the best available mechanism is the user provisioning REST API, which is best understood through the perspective of an administrator. This API is neither part of Jira itself, nor available to Forge or Connect Apps. It is part of the underlying Atlassian Identity platform.