Static path in custom folder?

I created a new project with --no-bunch. I wan’t to put some javascript files in their own custom folder and not in the customary “js” folder. So under priv/static I created a new folder “custom” alongside the “js” folder. I put a file inside it alert.js. So now I have

priv/static/custom/alert.js

and in the layout template I put this line

<script src="<%= static_path(@conn, "/custom/alert.js") %>"></script>

When I do this I get 404 for the file “alert.js”

When I move the file in the “js” folder

priv/static/js/alert.js

and

<script src="<%= static_path(@conn, "/js/alert.js") %>"></script>

it works fine. How to make static_path to also include the “custom” folder? I searched for static_path in source code but I admit I didn’t understand a lot.