Add a menu link to existing menu

Adding a menu link to some existing menu programmatically (for example in an update hook) is something I have to do fairly often. Here's how you can add a new menu link to the main menu:

use Drupal\menu_link_content\Entity\MenuLinkContent;

function MY_MODULE_update_8001() {
  MenuLinkContent::create([
    'title' => 'About',
    'link' => ['uri' => 'internal:/about'],
    'menu_name' => 'main',
    'weight' => -50,
  ])->save();
}

And if you want to add a submenu link you first have to load the parent link and then create a submenu link:

use Drupal\menu_link_content\Entity\MenuLinkContent;

function MY_MODULE_update_8001() {
  $menu_link_parents = \Drupal::entityTypeManager()
    ->getStorage('menu_link_content')
    ->loadByProperties([
      'title' => 'Dashboard',
      'menu_name' => 'main',
    ]);

  $menu_link_parent = reset($menu_link_parents);

  if ($menu_link_parent) {
    MenuLinkContent::create([
      'title' => 'Add tasks',
      'link' => ['uri' => 'internal:/add-task'],
      'menu_name' => 'main',
      'weight' => -50,
      'parent' => $menu_link_parent->getPluginId(),
    ])->save();
  }
}