add attribute to model

gistfile1.php

protected $appends = array(
        'date_range',
        'share_url'
    );

    public function getShareUrl()
    {

        $starts_at = Carbon::parse($this->starts_at);

        $share_url = '/calendar/' . $starts_at->year . '/' . $starts_at->month . '/#event/' . $this->id;

        return $this->attributes['share_url'] = $share_url;

    }