Example code for adding to the Event Espresso 4 Registrations admin list table

admin_reg_page.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_action(
    'AHEE__admin_wrapper__template__before_admin_page_content',
    'my_custom_add_to_admin_reg_list_table'
);
function my_custom_add_to_admin_reg_list_table() {
    $currentScreen = get_current_screen();
    if( 
        $currentScreen->id === "event-espresso_page_espresso_registrations" 
    ) {
        if (
            ! isset($_REQUEST['action']) || 
            empty($_REQUEST['action' ]) ||
            $_REQUEST['action'] === 'default'
        ) {
            // your code to add new things goes here
        } 
    }
}