Please does anyone know how I can add an add verification code to my forum header?
what do you mean with "verification code"?
You mean the one for example for webmaster tools?
If so, you have to add it to your theme index.template.php.
You have to write it according to php syntax, that's the only limitation.
If it is for Webmaster tool, download the verification file and upload it to the root directory of your domain