If the script is an HTML script tag like this:
<script src=https://vendor.com/js/script.js></script>Then you have two options:
1) Paste the tag into an HTML block in the specific box
This will download and run the script before the box is displayed. The downside of this approach is that you have to separately paste this into all boxes that require it, so can be tricky to maintain.
2) Add the script dynamically from Box JS