Custom Field to Link Title to External URL

I’m using the custom post hack to link the title of a post to an external link. The hack is #5 here: https://www.smashingmagazine.com/2009/05/13/10-custom-fields-hacks-for-wordpress/

I have changed my index.php and added the function to my functions.php

I’m running into an issue when trying to add the custom field, i get this error: Warning: Cannot modify header information – headers already sent by (output started at /home/shep/public_html/pieceofshep.com/wp-content/themes/elegant-box/functions.php:580) in /home/shep/public_html/pieceofshep.com/wp-includes/classes.php on line 1601

Any help would be appreciated!