code: func (u *Client) ClientCredentialGrant(clientId, clientSecret string) (string, error) {
  	values := url.Values{
  		"grant_type":    {"client_credentials"},
  		"response_type": {"token"},
  		"client_id":     {clientId},
  		"client_secret": {clientSecret},
  	}
  
  	token, err := u.tokenGrantRequest(values)
  
  	return token.AccessToken, err
  }
docs: // ClientCredentialGrant requests a token using client_credentials grant type