How to use ApplicationServlet to inject JQuery?

I’m trying to inject JQuery into a Vaadin app in order tu use it to modify the UI. However, I do not understand how or when to use the ApplicationServlet to inject the JQuery import into the HTML.

I’ve been looking at the following example:

Should I replace my Application extending class with this one? Or should the Application class call this?

This solution worked for me:
as the src-attribute value.