get dropdown selected value in Freemarker

Question

how to select the Drop-down selected value using free marker bellow is my code.

    <form action="" method="post">

    <@Table>
     <tr>
    <th>Year</th>
    <th>Month</th>
    <th>Supply unit</th>
    </tr>
    <tr>
    <td>
        <select id="year" name="year">
            <#list years as i>
                <option value="${i}">${i}</option>
            </#list>
        </select>
    </td>
    <td>
        <select id="month" name="month">
            <option value="">All</option>
            <#list months as j>
                <option value="${j}" >${j}</option>
            </#list>
        </select>
    </td>
    <td>
           <select id="supplyUnit" name="supplyUnit">
            <option value="">All</option>
            <#list supplyUnitMap?keys as su>
                <option value="${su}">${su}</option>
            </#list>
        </select>
    </td>
</tr>
</@Table>
<input type="submit" value="Download Statistics" class="Button" />

</form>

after selecting the Year and Month , it should automatically display start date and end date time of selected month.