多个表单不适用于商品上的FPDF和PHP POST

I rework the shop of a customer of mine, and he wants to generate a PDF of the offers he makes to his customers.
I have made the whole PDF template with FPDF and it works fine with a static variable with the offer number.
On the page are multiple forms, one for each offer. Each offer is generated with shopware, so i can't make different names for each submit button.

I need the value of the submit button or {$offerPosition.offerNumber} for the PHP.

The HTML form looks like this:

<form method="post" action="fpdf/pdf-creator.php" id="btn">
    <input type="submit" class="btn is--primary" value="{$offerPosition.offerNumber}">
</form>

And the PHP like this:

<?php

require('fpdf/fpdf.php');

$offernumber = substr($_POST['submit'], 8, 4);
$cn;
    // Instanciation of inherited class
    $pdf = new PDF();
    $pdf->AliasNbPages();
    $pdf->SetCreator();
    $pdf->SetTitle("Angebot Nr. " . $offernumber);
    $pdf->setOffernumber($offernumber);
    $pdf->setCustomernumber($cn);

    $pdf->AddPage();
    ...
    $pdf->Output();
?>

I hope somebody can help me.

Found the mistake. The submit had no name=""
It should look like this:

<input type="submit" name="submit" class="btn is--primary" value="{$offerPosition.offerNumber}">