Loading custom scripts in Squarespace

Squarespace is awesome, but its documentation is not. If you've got Developer Tools turned on in Squarespace, you can upload custom scripts into your /scripts directory and them import them via code injection.

Currently, the Squarespace docs give an example of using the Squarespace Script Loader:

<squarespace:script src="plugin.js" combo="true" />

...but I had no luck - my libraries weren't loading.

After more searching I found an old page for SS5 which shows a typical script load:

<script src="/scripts/plugin.js"></script>

Voila! It worked! 

Who knows why the SS loader isn't/wasn't working - we shouldn't be spending time on loading the scripts, we should be spending time on writing them.