code: func (s *AppsService) CreateInstallationToken(ctx context.Context, id int64) (*InstallationToken, *Response, error) {
  	u := fmt.Sprintf("app/installations/%v/access_tokens", id)
  
  	req, err := s.client.NewRequest("POST", u, nil)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	// TODO: remove custom Accept header when this API fully launches.
  	req.Header.Set("Accept", mediaTypeIntegrationPreview)
  
  	t := new(InstallationToken)
  	resp, err := s.client.Do(ctx, req, t)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return t, resp, nil
  }
docs: // CreateInstallationToken creates a new installation token.
  //
  // GitHub API docs: https://developer.github.com/v3/apps/#create-a-new-installation-token