We needed to add a field to a single woo product for the customer to input information. Claudeai had the answer with a mu-plugin.

CSS to style it:

/* RV Length field placeholder – faded color */
.my-custom-text-field input::placeholder { color: #aaa !important; opacity: 1 !important; }

/* RV Length text field – visible border */
.my-custom-text-field input[type=”text”] {
border: 1px solid #aaa !important;
border-radius: 8px !important;
padding: 8px 10px !important;
}

.my-custom-text-field input[type=”text”]:focus {
border-color: #0a12bc !important;
outline: none !important;
}

The mu-plugin:
<?php
/**

• Plugin Name: WooCommerce Product Custom Text Field
• Description: Adds a user-fillable text field to a specific WooCommerce product.
• Version: 1.0
  */

if ( ! defined( ‘ABSPATH’ ) ) {
exit;
}

// =============================================================================
// CONFIGURATION
// Set the product ID and field label here.
// To apply to ALL products remove the ID check in each function.
// =============================================================================
define( ‘MY_CUSTOM_FIELD_PRODUCT_ID’, 1234 ); // <– change to your product ID
define( ‘MY_CUSTOM_FIELD_LABEL’, ‘RV Length’ ); // <– change label as needed
define( ‘MY_CUSTOM_FIELD_PLACEHOLDER’, ‘How long is your RV?’ ); // <– optional placeholder
define( ‘MY_CUSTOM_FIELD_REQUIRED’, true ); // <– set true to make it required

// =============================================================================
// 1. Display the field on the product page
// =============================================================================
add_action( ‘woocommerce_before_add_to_cart_button’, function () {
add_action( ‘woocommerce_after_add_to_cart_form’, function () {
global $product;
if ( (int) $product->get_id() !== MY_CUSTOM_FIELD_PRODUCT_ID ) return;
?>
*

>
<?php
} );

// =============================================================================
// 2. Validate if required
// =============================================================================
add_filter( ‘woocommerce_add_to_cart_validation’, function ( $passed, $product_id ) {
if ( (int) $product_id !== MY_CUSTOM_FIELD_PRODUCT_ID ) return $passed;
if ( ! MY_CUSTOM_FIELD_REQUIRED ) return $passed;

if ( empty( $_POST['my_custom_text'] ) || trim( $_POST['my_custom_text'] ) === '' ) {
    wc_add_notice(
        esc_html( MY_CUSTOM_FIELD_LABEL ) . ' is required.',
        'error'
    );
    return false;
}
return $passed;

}, 10, 2 );

// =============================================================================
// 3. Save the value to the cart item
// =============================================================================
add_filter( ‘woocommerce_add_cart_item_data’, function ( $cart_item_data, $product_id ) {
if ( (int) $product_id !== MY_CUSTOM_FIELD_PRODUCT_ID ) return $cart_item_data;
if ( ! empty( $_POST[‘my_custom_text’] ) ) {
$cart_item_data[‘my_custom_text’] = sanitize_text_field( wp_unslash( $_POST[‘my_custom_text’] ) );
}
return $cart_item_data;
}, 10, 2 );

// =============================================================================
// 4. Display the value in the cart and checkout
// =============================================================================
add_filter( ‘woocommerce_get_item_data’, function ( $data, $cart_item ) {
if ( ! empty( $cart_item[‘my_custom_text’] ) ) {
$data[] = array(
‘name’ => esc_html( MY_CUSTOM_FIELD_LABEL ),
‘value’ => esc_html( $cart_item[‘my_custom_text’] ),
);
}
return $data;
}, 10, 2 );

// =============================================================================
// 5. Save the value to the order line item meta
// =============================================================================
add_action( ‘woocommerce_checkout_create_order_line_item’, function ( $item, $cart_key, $values ) {
if ( ! empty( $values[‘my_custom_text’] ) ) {
$item->add_meta_data(
esc_html( MY_CUSTOM_FIELD_LABEL ),
esc_html( $values[‘my_custom_text’] )
);
}
}, 10, 3 );