add_action('woocommerce_after_order_notes', 'custom_checkout_field');

function custom_checkout_field($checkout)

{

woocommerce_form_field('custom_field_name', array(
'type' => 'text',
'class' => array(
'my-field-class form-row-wide') ,

'placeholder' => __('New Custom Field') ,) ,
$checkout->get_value('custom_field_name'));
echo '</div>';

}