To implement Combo box also known as drop-down list, you should use <select> tag
<label for="cars">Choose a car:</label> <select name="cars" id="cars"> <option value="volvo">Volvo</option> <option value="saab">Saab</option> <option value="mercedes">Mercedes</option> <option value="audi">Audi</option> </select>
In case that you use PHP combined with select above, below example will be helpful.
$country_list = [
"LA",
"USA",
"AUS",
"BRA",
"CAN",
"CHN",
"EU",
"IND",
"JPN",
"RUS",
"SEA",
"TUR"
];
if (strlen($c)<1) $c="USA"; // BRA"; // need to get from parameters
echo '<label for="c">Choose a region:</label><select name="c" id="c">';
foreach($country_list as $v)
{
echo "<option value=\"{$v}\">{$v}</option>";
}
echo "</select>";