code: func (c *Client) CreateSignupToken(user services.UserV1, ttl time.Duration) (string, error) {
  	if err := user.Check(); err != nil {
  		return "", trace.Wrap(err)
  	}
  	out, err := c.PostJSON(c.Endpoint("signuptokens"), createSignupTokenReq{
  		User: user,
  		TTL:  ttl,
  	})
  	if err != nil {
  		return "", trace.Wrap(err)
  	}
  	var token string
  	if err := json.Unmarshal(out.Bytes(), &token); err != nil {
  		return "", trace.Wrap(err)
  	}
  	return token, nil
  }
docs: // CreateSignupToken creates one time token for creating account for the user
  // For each token it creates username and otp generator