code: func (u *Client) RefreshTokenGrant(clientId, clientSecret, refreshToken string) (string, string, error) {
  	values := url.Values{
  		"grant_type":    {"refresh_token"},
  		"response_type": {"token"},
  		"client_id":     {clientId},
  		"client_secret": {clientSecret},
  		"refresh_token": {refreshToken},
  	}
  
  	token, err := u.tokenGrantRequest(values)
  
  	return token.AccessToken, token.RefreshToken, err
  }
docs: // RefreshTokenGrant requests a new access token and refresh token using refresh_token grant type