magento2 – 404 error trying to use form in custom backend module

I am trying to use form POST method in my custom backend module from phtml, reloading the same page. But I get a 404. I have added form key. Been at it for days, so would appreciate some help.

Company/Logistikk/view/adminhtml/handleliste.phtml :

<?php 
    $action = (isset($_SERVER('HTTPS')) && $_SERVER('HTTPS') === 'on' ? "https" : "http") . "://$_SERVER(HTTP_HOST)$_SERVER(REQUEST_URI)";
    if(isset($_POST('test_string'))){
        $testString = $_POST('test_string');
        echo $testString;
    }
?>
<form name="test_form" method="POST" id="test-form" autocomplete="off">
  <input name="form_key" type="hidden" value="<?php /* @noEscape */ echo $block->getFormKey();?>" />
    <div class="admin__form-field-control">
        <input type="text" name="test_string" placeholder="Write something..." class="admin__control-text" action"<?php echo $action; ?>" />
    </div>
    <div class="admin__footer-main-actions">
        <input type="submit"  class="action-secondary" value="Send" />
    </div>
</form>

Company/Logistikk/Block/Logistikk.php :

<?php
namespace CompanyLogistikkBlock;

use MagentoBackendBlockWidgetContext;
use MagentoFrameworkDataFormFormKey;
use MagentoFrameworkViewElementTemplate;
class Logistikk extends Template
{
    protected $formKey;
    public function __construct(Context $context, FormKey $formKey, array $data = ())
    {
        $this->formKey = $formKey;
        parent::__construct($context, $data);
    }
    public function getFormKey()
    {
        return $this->formKey->getFormKey();
    }
}
?>

Company/Logistikk/view/adminhtml/layout/logistikk_handleliste_index.xml

<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <referenceBlock name="page.title">
        <action method="setPageTitle">
            <argument name="title" xsi:type="string">Company Handleliste</argument>
        </action>
    </referenceBlock>
    <body>
        <referenceContainer name="content">
            <block class="CompanyLogistikkBlockLogistikk" template="Company_Logistikk::handleliste.phtml"/>
        </referenceContainer>
    </body>
</page>