Getting a value from a drop down list using php

Hi, I am new to PHP and need some help.

I have a small Admin Panel to be used for various options. On the fist page I have a Dashboard, the content of the Dashboard will depend on the Selected Company. The Company is a Drop down list read from MySQL Database. That part works just fine.

<?php 
echo '<form method="post">';
$rows = db_select("SELECT idcompany, company FROM company order by idcompany;"); 
if($rows === false) {
		    $error = db_error();
			echo "<ul class='error'>";
			echo "<li class='error'>Error : $error </li> ";
			echo "</ul>";			
		}
?><label>Company: &nbsp; </label><?php 
?><select id="Company" onChange=" onChange="location.reload()"> <?php 
	 foreach($rows as $row){
          echo "<option value='" . $row['idcompany'] ."'>" . $row['company'] ."</option>";
	 } 
  echo '</select>';
  
echo '</form>'; 
?> 

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The issue is that depends on the Selected Company (company_id) the dashboard will display the content.

// SELECT * FROM notifications_vw where idcompany = $id_company

$id_company = $_GET['Company'];
$rows = db_select("SELECT * FROM notifications_vw where idcompany = $id_company"); 
if($rows === false) {
    $error = db_error();
	echo  $error;
}

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Unfortunately this does not work for me, I have a problem reading the list value. Ideally I should read it by the Tag ID or something like this.

The ID of the Company should be available at all times. I mean at the moment of the display of dashboard, add new customer, or display any other element that where idcompany is in use.

On the other hand when I change the Company from the drop down list, the website should reload and new company_id to be used.

Any ideas please?
List.PNG