Drupal 8 - View - Add a footer programmatically.

modulename.module.php

<?php
/**
 * Implements hook_views_pre_view().
 */
function projectname_views_pre_view(\Drupal\views\ViewExecutable $view, $display_id, array &$args) {
  // Project - Assets - 'Add' link.
  if ($view->id() == 'project_assets' && $display_id === 'block_1') {
    $account = \Drupal::currentUser();
    // If we're in a group.
    if (($group = \Drupal::routeMatch()->getParameter('group')) &&
      // And user has permissions to edit the group.
      ($group->hasPermission('edit group', $account))) {
      $options = array(
        'id' => 'area_text_custom',
        'table' => 'views',
        'field' => 'area_text_custom',
        'relationship' => 'none',
        'group_type' => 'none',
        'admin_label' => '',
        'empty' => TRUE,
        'tokenize' => FALSE,
        // Add an 'add' link for assets.
        'content' => \Drupal\Core\Link::fromTextAndUrl(
          t('Add'),
          \Drupal\Core\Url::fromUri(
            'internal:/group/' . $group->id() . '/content/add/group_node%3Alibrary_asset',
            array('query' => [\Drupal::destination()->getAsArray()  ])
          )
        )->toString(),
        'plugin_id' => 'text_custom',
      );
      $view->setHandler($display_id, 'footer', 'area_text_custom', $options);
    }
  }
}