Drupal.attachBehaviours() on Block that got rendered via ajax call
Ok, there are a few things that need to be done:


Create a custom ajax command
Create the JS to handle that custom command
Return an AjaxResponse from getWebformBlockHtml(), that contains that custom command from step 1
Ajaxify the response when your ajax is complete


You can do that as follows:

Step 1 - Create a custom ajax command

namespace Drupal\[MODULENAME]\Ajax;

use Drupal\Core\Ajax\CommandInterface;

class MODULENAMELoadFormCommand implements CommandInterface
{
  protected $form;

  public function __construct($form)
  {
    $this-&gt;form = $form;
  }

  /**
   * Implements Drupal\Core\Ajax\CommandInterface:render().
   */
  public function render()
  {
    return [
      'command' =&gt; 'MODULENAMELoadFormCommand',
      'form' =&gt; $this-&gt;form,
    ];
  }
}


Step 2 - Create the JS to handle that custom command

Add the following to your JavaScript that you're loading on page load:

Drupal.AjaxCommands.prototype.MODULENAMELoadFormCommand = function(ajax, response)
{
  $modal.find('.modal-content').first().html(response.form);
  $modal.foundation('open');
  Drupal.attachBehaviors($('#modal')[0], settings);
};


Step 3 - Return an AjaxResponse from getWebformBlockHtml() that contains that custom command from step 1

use Drupal\Core\Ajax\AjaxResponse;

public function getWebformBlockHtml($webformid) {
  $response = new Ajaxresponse();

  $webform = Webform::load($webformid);
  $renderable = $this-&gt;entityTypeManager()-&gt;getViewBuilder('webform')-&gt;view($webform);

  $rendered_form = \Drupal::service('renderer')-&gt;renderRoot($renderable);
  $response-&gt;setAttachments($renderable['#attached']);

  $response-&gt;addCommand(new MODULENAMELoadFormCommand($rendered_form));

  return $response;
}


Step 4 - Ajaxify the response when your ajax is complete

Modify your original Ajax Call to look like this:

$.ajax({
  url: '/contact_bridge/webform/' + targetForm,
  success:function(response) {
    var ajaxObject = Drupal.ajax({
      url: "",
      base: false,
      element: false,
      progress: false
    });

    // Simulate an AJAX response having arrived, and let the Ajax
    // system handle it.
    ajaxObject.success(response, "success");
  }
});