code: func (c *Client) AddonCreate(appIdentity string, plan string, options *AddonCreateOpts) (*Addon, error) {
  	params := struct {
  		Plan   string             `json:"plan"`
  		Config *map[string]string `json:"config,omitempty"`
  	}{
  		Plan: plan,
  	}
  	if options != nil {
  		params.Config = options.Config
  	}
  	var addonRes Addon
  	return &addonRes, c.Post(&addonRes, "/apps/"+appIdentity+"/addons", params)
  }
docs: // Create a new add-on.
  //
  // appIdentity is the unique identifier of the Addon's App. plan is the unique
  // identifier of this plan or unique name of this plan. options is the struct of
  // optional parameters for this action.