<link rel="javascript" href="script.js">

Is the line for you, I hope? (You can put this line in the section) What this does is search the folder that your .html file is inside of or the folders other children folders for "script.js", not the whole system sadly :( But is a good way to do this (and very simple)

I've just noticed before I hit post, that you've used this line already connecting your .css file to your .html file, it might have been different 10 years ago.

Please comment if this doesn't work :)