[html]
<form method="GET" action="https://duckduckgo.com/">
Insert your search terms here:&nbsp;&nbsp;&nbsp;
<input type="text" name="q" size="60">
<input type="hidden" name="sites" value="example.com">
<input type="submit" value="Search">
</form>
[/html]

or https://duckduckgo.com/search_box which seems to require JavaScript