Using data hook with URL field type in database

[_anno]

Hi, I’ve read that for a business to pay me every month for a link from my website it must not affect ranking so I should use a redirect that is blocked by robots.txt or “rel=nofollow”. I am looking to add the link using the URL field type; how would I add the rel=nofollow code? Ohad suggests I try using a data hook and add the suffix “rel=nofollow” whenever a URL is submitted, but I don’t know how to do that. Any help would be greatly appreciated.