Hi Bob, yes you can initiate a light box with iframe content from a link or button.
1. First, activate the iFrame option on Settings > Media. Once activated you may modify preferred light box sizes if you need to.
2. Then go to edit the page, post or the sidebar where you wish to create your link or button.
3. Insert an HTML block or widget and paste the following code:
<a href="#" class="fancybox-iframe button">LINK TEXT</a>
4. Replace the # in the href attribute with the URL that you would use in a normal iframe embed (the src attribute) and the LINK TEXT with whatever text you want to appear.
Save your post/page/widget and test 🙂
Note that the class “button” was added in the hope that it is a class that is defined in your theme stylesheet. If it’s not, you can open your theme Customizer and define it yourself in Custom CSS. See button example.