You can’t, we’ve been asking for the ability to reference our external file directories that we are using for our exported projects, but so far no love on that. The only way to do what you want to do is to use an Absolute URL for now and change it to a relative one when you’re done creating your project or are satisfied that all is working as it should with the script. I do this a lot with some scripts I use, once I know they work I don’t care if I can see them or not as long as I know they will work whenI export the site. Well I care, but, I don’t have much choice with them not giving us the ability to reference external directories.
The only other thing you can do is import the script or add a custom code block for it.