Custom Javascript Not Saving Config

I have a multisite implementation running Parabola.

I have a requirement to integrate each of my network sites with a chat feature (Rumbletalk). To do this I need to call their Javascript, but I want to add a different JS to each site (hence I can’t easily just modify the theme and put a single script in the header).

Within Parabola settings I notice that there is an option for ‘Custom Javascript’ but everytime I enter the script and then hit save, it refreshes and is empty.

Am I doing something wrong? – is there a particular way that you need to enter the script in to this box or does it just not work?

Many thanks

Mike