How do I add the data-toggle=”modal” data-target=”#myModal” attribures to a WP Nav bar?

PHP way:
Add the following to your functions.php file.
Filter nav_menu_link_attributes:

add_filter( 'nav_menu_link_attributes', 'my_menu_atts', 10, 3 );
function my_menu_atts( $atts, $item, $args )
{
  // Provide the id of the targeted menu item
  $menu_target = 123;

  // inspect $item

  if ($item->ID == $menu_target) {
    // original post used a comma after 'modal' but this caused a 500 error as is mentioned in the OP's reply
    $atts['data-toggle'] = 'modal';
    $atts['data-target'] = '#myModal';
  }
  return $atts;
}

jQuery way:

<script src="https://code.jquery.com/jquery-1.10.2.js"></script>
<script type="text/javascript">

    jQuery(document).ready(function() {
        jQuery('#menu-item-365').find('a').attr('data-toggle', 'modal');
        jQuery('#menu-item-365').find('a').attr('data-target', '#myModal');
    });
</script>