Woocommerce Custom Product Data Fields: Field setting to allow checkbox to be checked by default. https://wordpress.org/plugins/woocommerce-custom-product-data-fields/

your-field-init.php

$custom_product_data_fields[] = array(
      'id'          => '_mycheckbox',
      'type'        => 'checkbox',
      'label'       => __('Checkbox', 'wc_cpdf'),
      'description' => __('Field description.', 'wc_cpdf'),
      'desc_tip'    => true,
      'checked'     => true, // Allows the checkbox to be checked by default.
);