Prepopulate input field

I created a custom input field on a single woocommerce product page. The value of custom input field is saved into cart item data. Thats the code I put into functions.php:

add_action( 'woocommerce_before_variations_form', 'product_add_on', 9 );
function product_add_on() {
   global $product;
   if ( 12345 === $product->get_id() ) {
    $value = isset( $_POST['custom_text_add_on'] ) ? sanitize_text_field( $_POST['custom_text_add_on'] ) : '';
    echo '<div><label><span>Your Name <abbr class="required" title="required">*</abbr></span></label><input name="custom_text_add_on" value="' . $value . '">
</div>';
    }
}

Now I want to prepopulate this field with a Query-String. I want to send individual links to customers by email which include already the value of this field. Something like http://www.website.com/linktoproduct?customername=XYZ

Can anybody of you experts help me with this?

Thanks and regards!`