If you are provided a script that opens up a lightbox or pop-up, you will realize that you can not just paste it into the manual link field of a button in Enfold. These scripts usually create a plain text link. You can apply a class already used by Enfold to these scripts however to make them use for example the CSS for a button.
Here is the script I was provided from a company called Fareharbor:
<a href="https://fareharbor.com/cbstadventures/items/16545/">Book Now</a>
Here is it with a button class applied to the script:
<a class="avia-button avia-icon_select-no avia-color-light avia-size-large avia-position-center " href="https://fareharbor.com/cbstadventures/items/16545/">Book now!</a>
To see the result, click one of the Book Now buttons on this page: