WordPress – Add validation to custom adittional required checkout fields in WooCommerce

There are mistakes in your code, and your question is not very clear. For custom checkout required fields, you need to add a validation process.

Try the following revisited code:

add_action( 'woocommerce_before_order_notes', 'add_video_short_checkout_fields', 20, 1 );
function add_video_short_checkout_fields( $checkout ) {
    date_default_timezone_set('Europe/Amsterdam');

    // Loop through cart items
    foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
        $product_name = $cart_item['data']->get_name();

        // Check that 'my_custom_field_1' is set as custom cart item data
        if( isset($cart_item['my_custom_field_1']) ) {
            continue; // Jump to next cart item
        }

        // Restrict to 'video-short' product category cart items
        if ( has_term( 'video-short', 'product_cat', $cart_item['product_id'] ) ) {
            $current_date = date('d-m-Y', strtotime('+72 hours'));
            $hours_to_add = 24; // what is that for?

            // Add a form field for the current product
            echo '<div class="video-short-fields group_extra_info">
                <h3>' . esc_html( $product_name ) . '</h3>
                    <div class="video-short-fields form-row-group">';

            woocommerce_form_field( 'custom_field_1_' . $cart_item_key, array(
                'type' => 'text',
                'class' => array( 'video_veld customfield_start form-row-wide' ),
                'label' => __( 'Video Title Short', 'woocommerce' ),
                'placeholder' => __( 'Your title', 'woocommerce' ),
                'required' => true,
            ), $cart_item['my_custom_field_1'] );

            $hours_to_add += 24;  // what is that for?

            echo '</div></div>';
        }
    }
}

The field validation for custom checkout required fields:

// Field validation for custom checkout required fields
add_filter( 'woocommerce_checkout_process', 'custom_required_checkout_fields_validation' );
function disable_coupon_field_for_specific_products() {
    // Loop through cart items
    foreach ( WC()->cart->get_cart() as $item_key => $item ) {
        if( isset($_POST['custom_field_1_'.$item_key]) && empty($_POST['custom_field_1_'.$item_key]) ) {
            wc_add_notice( __('"Video Title Short" is a required  field.', 'woocommerce' ), 'error' );
        }
    }
}