Under Project Details is a space for entering a URL. Is it possible to add an additional boxes if there are multiple URL's that the project needs to reference for external information?

Thank you! I have added this to the custom form for the project.

There isn't a way to add additional boxes. I'd recommend using a custom form field, probably a paragraph text field, and storing all the URLs in there instead. If you already have a project custom form, your users are probably already referencing that for other things, so this would be also more efficient for them to go to the same place.