To submit HTML form by pressing ENTER key you can use following:

<form action="search.php" method="post">
<label>
<input id="searchVal" name="search" type="search" placeholder="Search Here..." 
onkeydown="if (event.keyCode == 13) document.getElementById('btnSearch').click()" />
</label>
<input id="btnSearch" class="btn btn-primary" type="button" value="Search" />
</form>