To make this, you have to create a facebook application to generate Facebook Access Token by following some steps:
Go to https://developers.facebook.com/ and click add a new app
Create a new App ID
After create app, go to Dashboard
Then Click show and Copy App Secret code.
Now go to your site admin panel. Then Extensions => plugins => Shortcode Ultimate => API key settings tab => FACEBOOK SETTINGS and paste app secret code.
After fill-up the blank field as per your Facebook App and finally click save button.