Add JS variable?

Hello,

I put an iFrame on the page that contains a form

https://internetwerkbon.nl/abonneer/

Now I would like the

function calculateTotal()
{
    var gebruikersAantal = getAantal() * getMaanden();

    var divobj = document.getElementById('totaalPrijs');
    divobj.style.display='block';
    divobj.innerHTML = "Totaalprijs €"+gebruikersAantal;
}

That’s being displayed in
<div id="totaalPrijs"></div>

How do I add the value that appears between the DIV to be sent with the form?

Thank you.

https://www.ads-software.com/extend/plugins/contact-form-7/