Loading server side files efficiently

I’m looking for some advice on the best way to carry out a task I’ve been given. Long story short, I’ve recently inherited an old web chat client that consists mainly of 2 JavaScript files that are targeted based on whether or not the chat button is enabled. In the scenario where the button is enabled, this is clicked and it opens a new window and allows you to proceed with the chat, however it’s not very user friendly and still uses tabling for layouts. So far, I’ve managed to strip all that down and use bootstrap to create a nice modal that pops out and allows the same functionality but this now works for mobile. Although, here’s my issue. I’ve managed to get this working by adding it directly to the index.html file (something I wouldn’t be able to do in production) so what I’m looking to do, is when that button is clicked, load the modal from my web server instead along with the style sheets and scripts.

I’m just looking for suggestions on the best ways to do this or some pointers. Happy to answer any queries as well.

Kindest regards and thank you in advance

E