Form in Lightbox
You can display the frontend submission form inside a popup/lightbox using a Lightbox plugin. In this example, I used the free Easy Fancybox, but most lightbox plugins will have similar features.
After you install Easy Fancybox, in the Settings > Media page, enable the option ‘Inline Content’ in the Fancybox options.
Once you have this enabled, you can create your Frontend Submission Form shortcode in Testimonials > Shortcode Generator > FrontEnd Submission Form.
Once you have your shortcode, you can create/edit the page where you want the form in the lightbox to display and add the following in the content of the page/post, in the ‘Text’ separator:
<a class="fancybox" href="#testimonialform">Add New Testimonial</a> <div class="fancybox-hidden" style="display: none;"> <div id="testimonialform" >[show-testimonials-form alias='submit_lightbox' ]</div> </div>
You should edit the text and the shortcode for your form. This will output the following link, that you can click and a lightbox with the form will display: