1. create a new node in acp (your board url/admin.php?nodes/) and select "page".
2. Type the message in "Template-HTML" (disable "Display in the node list") and save it. Remember the URL portion.
The page url = your board url/index.php?pages/URL portion/ and the content will be show in the modal (overlay).
3. Set the button: <a href="your board url/index.php?pages/URL portion/" data-xf-click="overlay">
<button class="actionBar-action actionBar-action--reply"> Button text</button></a>