do_action( "wp_ajax_{$action}" )

More Information

• This hook allows you to handle your custom AJAX endpoints. The wp_ajax_ hooks follows the format “wp_ajax_$action“, where $action is the ‘action‘ field submitted to admin-ajax.php.
• This hook only fires for logged-in users. If your action only allows Ajax requests to come from users not logged-in, you need to instead use wp_ajax_nopriv_$action such as: add_action( 'wp_ajax_nopriv_add_foobar', 'prefix_ajax_add_foobar' );. To allow both, you must register both hooks!
• See also wp_ajax__requestaction
• See also Ajax Plugin Handbook