You should have already followed the Capturing and displaying information using custom meta boxes recipe to have a starting point for this recipe and the resulting plugin should still be active in your development site. Alternatively, you can get the resulting code (Chapter 5/ch5-post-source-link/ch5-post-source-link-v1.php) from the code bundle you downloaded from the Packt Publishing website (https://www.packtpub.com/support) and rename the file as ch5-post-source-link.php.