<tr><td>Brand:  </td><td>   <select style='width:205px' name="BrandBox" onchange='otherbrand(this.val)'>

<?php

$qry = "Select BrandName from Brand";
$populate = mysqli_query($conn, $qry);

while ($run = mysqli_fetch_assoc($populate)){
echo "<option value="'.$run['BrandName'].">'".$run['BrandName']."</option>";
}

?>
<option value="Other">Other</option></td>


<script type="text/javascript">
function otherbrand(val){
var element = document.getElementById('otherb');
if(val == 'Other')
element.style.display='block';
else
element.style.display='none';
}
</script>

<td><input type="text" name="BrandBox" id="otherb" style='display:none'/></td>
</select></tr>