XF 1.5 how to show display results as threads in searchbox

[swatme]

Hi

how to set the search function to have "Display Results as threads"

this is the code for searchbox, but it shows the posts and threads.
i want it to be threads only

<form action="{xen:link 'search/search'}" method="post" style="width: 100%;" >
<input type="text" name="keywords" id="s"  inputmode="predictOn" placeholder="type here?" value="" />
<input type="submit" style="padding: 6px;" value="Search" />
<input type="hidden" name="_xfToken" value="{$visitor.csrf_token_page}" />
</form>

thanks