//* Add select field to the checkout page
add_action('woocommerce_before_order_notes', 'wps_add_select_checkout_field');
function wps_add_select_checkout_field( $checkout ) {
    echo '<h2>'.__('How did you hear about us?').'</h2>';
    woocommerce_form_field( 'hearabout', array(
        'type'          => 'select',
        'class'         => array( 'wps-drop' ),
        'options'       => array(
            'blank'     => __( 'Select an option', 'wps' ),
            'google search' => __( 'Google Search', 'wps' ),
            'social media'  => __( 'Social Media', 'wps' ),
            'friend or family'  => __( 'Friend or family', 'wps' ),
            'other'     => __( 'Other', 'wps' )
        )
 ),
    $checkout->get_value( 'hearabout' ));
}

//* Process the checkout
 add_action('woocommerce_checkout_process', 'wps_select_checkout_field_process');
 function wps_select_checkout_field_process() {
    global $woocommerce;
    // Check if set, if its not set add an error.
    if ($_POST['hearabout'] == "blank")
     wc_add_notice( '<strong>Please select where you heard about us.</strong>', 'error' );
 }

//* Update the order meta with field value
 add_action('woocommerce_checkout_update_order_meta', 'wps_select_checkout_field_update_order_meta');
 function wps_select_checkout_field_update_order_meta( $order_id ) {
   if ($_POST['hearabout']) update_post_meta( $order_id, 'hearabout', esc_attr($_POST['hearabout']));
 }

//* Display field value on the order edition page
add_action( 'woocommerce_admin_order_data_after_billing_address', 'wps_select_checkout_field_display_admin_order_meta', 10, 1 );
function wps_select_checkout_field_display_admin_order_meta($order){
    echo '<p><strong>'.__('Heard about us at').':</strong> ' . get_post_meta( $order->id, 'hearabout', true ) . '</p>';
}
//* Add selection field value to emails
add_filter('woocommerce_email_order_meta_keys', 'wps_select_order_meta_keys');
function wps_select_order_meta_keys( $keys ) {
    $keys['Hearabout:'] = 'hearabout';
    return $keys;

}