Custom Fields

How can I pull in the values from my custom field in a Page to be shown here:

<form method="get" id="search form" action="/">
<div>
<input type="text" value="" name="s" id="s" />
<input type="hidden" value="CUSTOM FIELD VALUE HERE" name="cat" id="scat" />
<input type="submit" id="search_submit" name="Search" value="Search"/>
</div>
</form>

Ultimately, I am trying to have a dynamic search to query from within a tag or set of tags that are defined in the page’s custom fields (related_tags).