Php filter database record using dropdownlist

<select class="custom-select" name="cbodept" id="cbodept-id" style="width: 50%;" >
    <option value="" selected> - SELECT DEPARTMENT - </option>
    <?php // Fetch|Load records to dropdownlist
        include "../php-file/db-config.php";
        $sql = "SELECT  DISTINCT deptcode_fk FROM tblcourse ORDER BY deptcode_fk ASC";
        $stmt = $conn -> prepare($sql);
        $stmt -> execute();
        $result = $stmt -> get_result();
        while($row = $result -> fetch_assoc()){
            #<option value="">- display -</option>
            echo " <option value=' ". $row['deptcode_fk'] ." '> " .$row['deptcode_fk'] ." </option>" ;
        }
        // Close
        $stmt -> close();
        $conn -> close(); 
    ?>
</select> 

<pre><?php  //DISPLAY RECORD
    if(isset($_POST['btnsearch'])){
        $value = $_POST['cbodept']; #this line not working
        include "../php-file/db-config.php";
        $sql = "SELECT * FROM tblcourse WHERE deptcode_fk=? ORDER BY deptcode_fk ASC";
        $stmt = $conn -> prepare($sql);
        $stmt -> bind_param("s",$value);
        // Execute and fetch
        $stmt -> execute();
        $result = $stmt -> get_result();
        // Echo
        while($row = $result -> fetch_assoc()){
            echo "<tr >
                <td > ".$row['crscode']." </td>
                <td > ".$row['deptcode_fk']." </td>
                <td > ".$row['crsname']." </td>
                <td > ".$row['duration']." </td>
                <td > ".$row['crsstat']." </td>
                <td class='action-buttons'> <a href='#updDept.php?id=".$row['crscode']."' value='Edit'> Edit </a> </td>
                <td class='action-buttons'> <a onClick=\"javascript: return confirm('Please confirm deletion of:  ".$row['crscode']." ?');\" href='#delDept.php?id=".$row['crscode']."' value='Del'> Del</a> </td>
                <td hidden class='action-buttons'> <a href='#printDept.php?id=".$row['crscode']."' target ='_self' value='N/A'> N/A</a> </td>                                   
            </tr>";
        }
        // Close
        $stmt -> close();
        $conn -> close(); 
    }
?>