code: func NewWithUserServiceKey(key string, opts ...Option) (*API, error) {
  	if key == "" {
  		return nil, errors.New(errEmptyCredentials)
  	}
  
  	api, err := newClient(opts...)
  	if err != nil {
  		return nil, err
  	}
  
  	api.APIUserServiceKey = key
  	api.authType = AuthUserService
  
  	return api, nil
  }
docs: // NewWithUserServiceKey creates a new Cloudflare v4 API client using service key authentication.