Refer to the guide from Facebook:
Steps in a nutshell
- Create your Facebook App.
- Make sure you’re the admin of the App and the Page
- Add Your App to your Facebook Page.
Adding your App to a Page
This step can be a bit tricky, so read carefully. In order to install your new page tab on your business page, you need to visit a link with several custom URL parameters. The link is:
You’ll need to replace the parts I’ve bolded in the URL above — “YOUR_APP_ID” and “YOUR_URL” with some information that Facebook provides for your app, your “App ID/API Key” and your “Site URL.”
After using the URL, you will see a dialog box asking you to choose which Page you want to add the App to. Select the Page and click Save.