sample of adding upsell order notes

upsell_order_notes.php

<?php

add_action('wc_1cu_after_item_add', 'update_order_meta_after_add', 10,1);
function update_order_meta_after_add($order){
    if(!empty($_GET['offer_notes']) ){
        //assume the last order item added.
        $items = $order->get_items();
        end($items);
        $item_id = key($items);
        wc_add_order_item_meta( $item_id, 'upsell_order_notes', sanitize_text_field($_GET['offer_notes']));
    }
}

add_shortcode( 'upsell_order_notes', 'upsell_order_notes_func' );
function upsell_order_notes_func($atts){

    if( 
        empty( $_GET['1cu'] ) || empty( $_GET['1cu_s'] ) 
    ){
        return;
    }

    ob_start();
    
    ?>
    <script>
    jQuery(function($){
     $('.wc_1cu_custom_offer_accept, .wc_1cu_default_offer_accept').hide();
     $('#offer_notes_block').find('input[type="submit"]').on('click', function(){
       var url = $('a.wc_1cu_custom_offer_accept').attr('href');
       var offer_notes = $('#offer_notes_block').find('input[name="offer_notes"]').serialize();
       window.location = url + '&' + offer_notes;
       return false;
     })
    });
    </script>
    <p id="offer_notes_block">
       <label for="offer_notes">Notes for Your Designer</label>
       <input name="offer_notes" type="text" placeholder="">
       <input type="submit" value="Accept Offer">
    </p>
    <?php
    
    return ob_get_clean();
}