Make a new template with all the HTML in it... then inside your forum root 'global.php' script, find this :
PHP Code:
eval("\$headinclude = \"".gettemplate('headinclude')."\";");
eval("\$header .= \"".gettemplate('header')."\";");
... and above that, put this :
PHP Code:
eval("\$thingy_template = \"".gettemplate('thingy_template')."\";");
... substituting the 'thingy_template' bits with whatever you call the template you create.
Then, go into your 'header' template, and place :
$thingy_template, wherever you want the template to appear.
Of course, this is assuming you want this template parsed for every page.
Specify the single page if you only want to display it on one page.