[WordPress] Creating Custom WooCommerce Input Field

00-init.php

<?php

public function init() {

	add_action(
		'woocommerce_product_options_grouping',
		array( $this, 'product_options_grouping' )
	);

	add_action(
		'woocommerce_process_product_meta',
		array( $this, 'add_custom_linked_field_save' )
	);
}

01-custom-input.php

<?php

public function product_options_grouping() {

	// Generic, I know.
	$description = sanitize_text_field(
		'Enter information relevant to the customer of this product.'
	);

	$args = array(
		'id' 		=> 'acme_text_id',
		'label'		=> sanitize_text_field( 'Product IDs' ),
		'placeholder'	=> '',
		'desc_tip'	=> true,
		'description'	=> $description,
	);
	woocommerce_wp_text_input( $args );
}

02-saving-input.php

<?php

public function add_custom_linked_field_save( $post_id ) {

	if ( ! ( isset( $_POST['woocommerce_meta_nonce'], $_POST[ 'acme_text_id' ] ) || wp_verify_nonce( sanitize_key( $_POST['woocommerce_meta_nonce'] ), 'woocommerce_save_data' ) ) ) { // Input var okay.
		return false;
	}

  	// This is where you can take it a step further to add your own validation.
	$ids = sanitize_text_field(
		wp_unslash( $_POST['acme_text_id'] ) // Input var okay.
	);

	update_post_meta(
		$post_id,
		'acme_text_id',
		esc_attr( $ids )
	);
}