Place the add to cart link right on the registration page. Works with Event Espresso 3 and Multi Event Registration.

single-event-cart-mer.php

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

add_action('action_hook_espresso_social_display_buttons', 'single_custom_cart_link');

function single_custom_cart_link($event_id) {
    global $wpdb;
    $event_name = $wpdb->get_var("SELECT event_name FROM " . EVENTS_DETAIL_TABLE . " WHERE id='" . $event_id . "'");

    ?><div class="custom-cart-link" style="display:block;"> 

        <?php echo do_shortcode('[ESPRESSO_CART_LINK event_id="'.$event_id.'" event_name="'.$event_name.'" anchor="Add this session to your cart"]'); ?>
    
    </div>
    <?php
}