Right now the Content Box Element URL field will not open a modal, we have a Git item for this, for our developers to building into Elements like the Content Box.
You still can do this with a small workaround, using a button element, added to the Content Box text area. Add a content box, add a modal to the page and then edit the content box and using the FB selector add a button with the modal name added to the ‘Nodal Window Anchor’ field -> https://d.pr/i/z0ZYuj
Here is example raw, add it to a test page on your site to play with https://gist.github.com/m1chelangel0/1b0e4c997cb918c9528d19c12b0dbab8
Thank you kindly