Add custom field to the checkout page

add_action('woocommerce_after_order_notes', 'custom_checkout_field');

function custom_checkout_field($checkout)

{

echo '<div id="custom_checkout_field"><h2>' . __('New Heading') . '</h2>';

woocommerce_form_field('custom_field_name', array(

'type' => 'text',

'class' => array(

'my-field-class form-row-wide'

) ,

'label' => __('Custom Additional Field') ,

'placeholder' => __('New Custom Field') ,

) ,

$checkout->get_value('custom_field_name'));

echo '</div>';

}