How to use Drupal 8 ajax API for Menu links I have a taxonomy menu on the left side of the screen. When I select a menu item I want the results to appear in the main content area of the screen. 

I want this to happen using AJAX. I have been reading the AJAX API documentation from Drupal:
https://api.drupal.org/api/drupal/core%21core.api.php/group/ajax/8.4.x

It says I can add a "use-ajax" class to the link, which I have. But then there is no documentation regarding how to get the link to load in a target ID.
You can try this for adding 'use-ajax' class to menu link:

/**
 * Implements hook_menu_links_discovered_alter().
 */
function MYMODULE_menu_links_discovered_alter(&amp;$links) {
  if (isset($links['MENU_LINK_ID'])) {
    $links['MENU_LINK_ID']['options']['attributes']['class'][] = 'use-ajax';
    $links['MENU_LINK_ID']['options']['attributes']['data-dialog-type'] = 'modal';
  }
}