code: func (c *Client) AddHerokuKey(key string) error {
  	body := struct {
  		APIKey string `json:"apikey"`
  	}{APIKey: key}
  
  	return c.request("POST", "/user/heroku-key", nil, nil, body)
  }
docs: // AddHerokuKey associates a Heroku key with the user's API token to allow
  // CircleCI to deploy to Heroku on your behalf
  //
  // The API token being used must be a user API token
  //
  // NOTE: It doesn't look like there is currently a way to dissaccociate your
  // Heroku key, so use with care