html formatted email using form api
Same thing i do in my module may be it is set to you requirement 

    global $content;
    $content .= '&lt;button class="btn btn-primary" data-toggle="modal" data-target=".bd-example-modal-lg"&gt;Apply&lt;/button&gt;';
    $content .= '&lt;div class="modal fade bd-example-modal-lg" tabindex="-1" role="dialog" aria-labelledby="myLargeModalLabel" aria-hidden="true"&gt;';
    $content .= '&lt;div class="modal-dialog modal-lg"&gt;';
    $content .= '&lt;div class="modal-content"&gt;';
    $content .= drupal_render($form);
    $content .= '&lt;/div&gt;';
    $content .= '&lt;/div&gt;';
    $content .= '&lt;/div&gt;';
    return $content;