code: func (c *Client) GetSignupU2FRegisterRequest(token string) (u2fRegisterRequest *u2f.RegisterRequest, e error) {
  	out, err := c.Get(c.Endpoint("u2f", "signuptokens", token), url.Values{})
  	if err != nil {
  		return nil, err
  	}
  	var u2fRegReq u2f.RegisterRequest
  	if err := json.Unmarshal(out.Bytes(), &u2fRegReq); err != nil {
  		return nil, err
  	}
  	return &u2fRegReq, nil
  }
docs: // GetSignupU2FRegisterRequest generates sign request for user trying to sign up with invite tokenx