Well, it is called "Custom HTML..." and not "Custom PHP...", right? :)
In order to include PHP code it is best to modify whatever theme you're using. So open the qa-theme/<your_selected_theme>/qa-theme.php file. As you are talking about "bottom of every page" then you will need to override the body_footer function.
If it is there, edit it, if it isn't add it:
public function body_footer() {
if (isset($this->content['body_footer']))
$this->output_raw($this->content['body_footer']);
$somethingToOutput = "Bleh, I'm in the bottom";
$this->output_raw($somethingToOutput);
}
And that's it.