In Expressionengine, if you want to make a custom field work as a URL / link instead of plain text, under channel preferences for the required channel ensure ‘Automatically turn URLs and email addresses into links?’ is set to NO.
Otherwise the link on the page will add the URL of the page you are currently on e.g example.com/url-of-custom-field which of course won’ t work.
This catches me out often enough that I need to document it here.