routes – How to add custom URL for node/add/{custom-content-type} in Drupal 8?

I have custom content type which I fill up with the help of Multistep Form(used contributed module called Froms Stpes).

Now I want to set custom url for Node/add/{custom-content-type}, I tried to use alterRoute method in RouteSubscriber as below:

   * {@inheritdoc}
  protected function alterRoutes(RouteCollection $collection) { 
     if ($route = $collection->get('node.add_page')) {
         $current_uri = Drupal::request()->getRequestUri();
         if($current_uri=="node/add/custom-content-type") {

But above code is not changing the URL, also I am finding it difficult to apply condition that URL should change only for custom content type.

Please Help