Adding External Script Plugin

Hi I need to add external plugins such as crisp plugin, widget and other plugins. The instruction was to add it in the header of an html file but I don’t know where I can add it in my code.

Which page do you want these scripts to be added?
You can use to inject scripts to desk and other standard pages.

I’ve done some research and it was said you could add it in app_include_js in
My next questions is can you add multiple javascript in app_include_js? And if yes, is this applicable for all pages?