problem with adding leaderboard

[Ripon Ahmed]

Hello everyone, I am new in phaser 3 game development . I am trying to build a social game for the Facebook Instant Games platform. I create a simple phaser 3 game and now I want to add leaderboard with my game . when add this code my game

this.facebook.on('getleaderboard', function (leaderboard)
        {
            this.leaderboard = leaderboard;

        }, this);

        this.facebook.getLeaderboard('my_leaderboard');
 this error message show the console  Uncaught TypeError: Cannot read property 'on' of undefined

please any one can help me. Thanks everyone.