Adding JavaScript in LivePerson's chat window

I tried to ask this question in the liveperson's technical discussion section:

Adding JavaScript in LivePerson's chat window

However, nobody seems to answer the question yet.

One of liveperson's chat representative recommends me to ask this question in the developer network instead.

To reiterate the question:

I would like to add an external JavaScript in the pop up window (ie: https://sales.liveperson.net/hc... window).

More specifically, I'd like to add the JavaScript in both pre chat window, as well as the window where the actual chat occurs.

What would be the best way to do this?

I would appreciate any advice,

Thanks.