Help add featured image external link custom field

I tried adding an external link to a featured image using code at http://wordpress.org/support/topic/how-to-add-an-external-url-to-a-featured-image?replies=19 but it’s not working at all.

I found ACF and I think that I can add an external link to a featured image using this plugin’s custom field. Could someone help me get the featured image external link working?

http://wordpress.org/plugins/advanced-custom-fields/