How could I send information without lose another information posted?

I have two pages:

1- CompanySelect.php

2- CompanyData.php

The page 1 gives a list of options to select:

echo'<form action="companyList.php" method="post">';

             echo'<select name="ID_Client">'; //That`s the Referenced column

             echo'<option value="0" label="coyvalue" selected="selected"> Select Company </option>';


             while($row = mysql_fetch_array($tIDENTITY_Execution, MYSQL_ASSOC)){
                 $ID_Client = $row['ID_Client'];
                 $Comp_Name = $row['Comp_Name'];

                 echo'<option value= "'.$ID_Client.'">'.$Comp_Name.'</option>';
             }

             mysql_free_result($tIDENTITY_Execution);



             echo'</select>';

             echo'<input type = "submit" />';

             echo '</form>';

The page 2 shows information about the Company Selected. I would like to add a link in one of the columns to ORDER BY DATE. Please se below part of the code:

//Using Get to order the list
$order = $_GET["order"];

//If any order selected, default = Last Communication Date
if(!isset($order)){
    $order = "Last_Communication_Date";
}
enter code here
$tsales_cmd = " SELECT *";
$tsales_cmd .= " FROM tsales_history ";
$tsales_cmd .= " WHERE tIDENTITY_ID_Client = '".$companyID."' ";
$tsales_cmd .= " ORDER BY ";
$tsales_cmd .= " tsales_history.".$order;
$header_lastComm = '<a href = "'.$thisScriptName.'?order=Last_Communication_Date"><span class = "tableHeader"> LAST COMM </span> </a>';

...

echo "<table border = '1' padding = '2'>";
echo '<tr>
            <td> Average Order </td>
            <td> Last Order Date </td>
            <td> '.$header_lastComm.'</td>
            <td> Product Bought </td>
            <td> Services Bought </td>
            <td> After Sale Service </td>
            <td> Market Share </td>
      </tr>  ';

However, I am losing the ID Selected. How could I fix this problem?