To make this, you have to create a Facebook application to generate Facebook App ID 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 Copy App ID
Now go to your site admin panel. Then Extensions => plugins => Shortcode Ultimate => API key settings tab => FACEBOOK SETTINGS and paste App ID.
After fill-up the blank field as per your Facebook app and finally click save button.