Hello,
If you do not set the price for the product “Add to cart button” would not appear.
If you want to show the popup on selected products:
1. Create HTML block;
2. Add the Popup element and the content as per your needs: insert the contact form inside.
3. Insert the shortcode of this HTML block in the bottom of the short description of the product.
If you want to show this popup on all the products, insert this HTML shortcode in the Before add to cart area in the Theme Settings > Product page.
Best Regards