add_action( 'woocommerce_product_options_general_product_data', 'wc_custom_add_custom_fields' );

add_action( 'woocommerce_product_options_general_product_data', 'wc_custom_add_custom_fields' );
function wc_custom_add_custom_fields() {
    // Print a custom text field
    woocommerce_wp_text_input( array(
        'id' => '_my_text_field',
        'label' => 'Lisaväli',
        'description' => 'This is a custom field, you can write here anything you want.',
        'desc_tip' => 'true',
        'placeholder' => 'Sisesta kogus'
    ) );
}

add_action( 'woocommerce_process_product_meta', 'wc_custom_save_custom_fields' );
function wc_custom_save_custom_fields( $post_id ) {
    if ( ! empty( $_POST['_my_text_field'] ) ) {
        update_post_meta( $post_id, '_my_text_field', esc_attr( $_POST['_my_text_field'] ) );
    }
}
// Show on frontend
add_action( 'woocommerce_single_product_summary', 'print_my_cf_value', 20 );
function print_my_cf_value() {
    global $product;
    echo '<p>' . esc_html( get_post_meta( $product->ID, '_my_text_field', true ) ) . '</p>';
}