Use Case/Examples for Custom header plugin for OJS

i already use the Custom header plugin and its very awesome… I can put all code ( mix HTML,CSS, and Java
to create an image slider/slideshow here is the example :

I am trying to execute some javascipt using the Custom Header plugin, but without any success so far!
We are using OJS
Direct code, i.e.
<script> alert('hello'); </script>
is just not accepted, apparently. The plugin window keep procesing, no errors, but just never take it.
Loading an external script is accepted immediately by the plugin
<script src="http://my/site/hello.js"></script>
In the page source the script appears as loaded.
The code inside is simple and visible:
…but it is not executed…

Of course, this code is only for troubleshooting to identify the problem.

I have read about the Custom header plugin and apparently it makes possible to introduce any type of code…

Any idea of what is my mistake?

Thanks in advance!