How to write hook_field_placeholder_info() for the link module?
There is a feature request to add support for the link module. Unfortunate that patch has not been committed yet, but you can use/apply the patch in #5 and use

/**
 * Implements hook_field_placeholder_info().
 */
function mymodule_field_placeholder_info() {
  return array(
    'link_field' =&gt; 'url',
  );
}