Creating field on each node
If you have a link field added in your content type then there is no need to render the field programmatically. As I understand with your comment, you need to add field value programmatically.  So in drupal 7 it was hook_node_insert and in drupal 8 it is hook_ENTITYTYPE_insert. 

So in drupal 8 your hook_node_insert will look like following :

function hook_node_insert(Drupal Core Entity EntityInterface $node) {
  // Change node object how you wanted.
}


Helpful Links :

https://www.drupal.org/node/2294409

http://www.drupalcontrib.org/api/drupal/drupal!core!modules!node!node.api.php/function/hook_node_insert/8