Show Custom Input Field @ WooCommerce Single Product Page

functions.php

<?php
// Source: https://businessbloomer.com/woocommerce-product-add-ons-without-plugin/?ck_subscriber_id=69986993
// -----------------------------------------
// 1. Show custom input field above Add to Cart
 
add_action( 'woocommerce_before_add_to_cart_button', 'bbloomer_product_add_on', 9 );
 
function bbloomer_product_add_on() {
    $value = isset( $_POST['_custom_text_add_on'] ) ? sanitize_text_field( $_POST['_custom_text_add_on'] ) : '';
    echo '<div><label>Custom Text Add-On <abbr class="required" title="required">*</abbr></label><p><input name="_custom_text_add_on" value="' . $value . '"></p></div>';
}
 
// -----------------------------------------
// 2. Throw error if custom input field empty
 
add_filter( 'woocommerce_add_to_cart_validation', 'bbloomer_product_add_on_validation', 10, 3 );
 
function bbloomer_product_add_on_validation( $passed, $product_id, $qty ){
    if( isset( $_POST['_custom_text_add_on'] ) && sanitize_text_field( $_POST['_custom_text_add_on'] ) == '' ) {
        wc_add_notice( 'Custom Text Add-On is a required field', 'error' );
        $passed = false;
    }
    return $passed;
}
 
// -----------------------------------------
// 3. Save custom input field value into cart item data
 
add_filter( 'woocommerce_add_cart_item_data', 'bbloomer_product_add_on_cart_item_data', 10, 2 );
 
function bbloomer_product_add_on_cart_item_data( $cart_item, $product_id ){
    if( isset( $_POST['_custom_text_add_on'] ) ) {
        $cart_item['custom_text_add_on'] = sanitize_text_field( $_POST['custom_text_add_on'] );
    }
    return $cart_item;
}
 
// -----------------------------------------
// 4. Display custom input field value @ Cart
 
add_filter( 'woocommerce_get_item_data', 'bbloomer_product_add_on_display_cart', 10, 2 );
 
function bbloomer_product_add_on_display_cart( $_data, $cart_item ) {
    if ( isset( $cart_item['custom_text_add_on'] ) ){
        $data[] = array(
            'name' => 'Custom Text Add-On',
            'value' => sanitize_text_field( $cart_item['custom_text_add_on'] )
        );
    }
    return $data;
}
 
// -----------------------------------------
// 5. Save custom input field value into order item meta
 
add_action( 'woocommerce_add_order_item_meta', 'bbloomer_product_add_on_order_item_meta', 10, 2 );
 
function bbloomer_product_add_on_order_item_meta( $item_id, $values ) {
    if ( ! empty( $values['custom_text_add_on'] ) ) {
        wc_add_order_item_meta( $item_id, 'Custom Text Add-On', $values['custom_text_add_on'], true );
    }
}
 
// -----------------------------------------
// 6. Display custom input field value into order table
 
add_filter( 'woocommerce_order_item_product', 'bbloomer_product_add_on_display_order', 10, 2 );
 
function bbloomer_product_add_on_display_order( $cart_item, $order_item ){
    if( isset( $order_item['custom_text_add_on'] ) ){
        $cart_item_meta['custom_text_add_on'] = $order_item['custom_text_add_on'];
    }
    return $cart_item;
}
 
// -----------------------------------------
// 7. Display custom input field value into order emails
 
add_filter( 'woocommerce_email_order_meta_fields', 'bbloomer_product_add_on_display_emails' );
 
function bbloomer_product_add_on_display_emails( $fields ) { 
    $fields['custom_text_add_on'] = 'Custom Text Add-On';
    return $fields; 
}