Ad blocker messsage

Can anyone help me out with one of two things?

I want to leave a message for people who are using ad-blockers to please disable them on the forum. I don't want to prevent them from using the forum if they have.

I looked into it and can see how to do it in the HTML source code which I can see when I use "Inspect (Q)" in Firefox but I don't know where this HTML file is in the installation directory.

1. Can someone point out how to find and edit the HTML?

2. Is there a plug-in which does the job? (leave a message asking users to disable ad-block). I know there is a plug in which prevents people from seeing the page at all until they disable the ad block. I don't want to go that far.