I'd like to submit a form with a custom variable from the `prompt()` function.

Submitting a form uses the code

```
document.formname.submit();

```

But that will just submit the form, what I want is:

```
var iInvoiceID = prompt("Please enter the invoice ID");
document.formname.submit(); // But it should send iInvoiceID within $_POST aswell

```

One way could be: after the prompt, set an input type = 'hidden' to `iInvoiceID`'s value, and then submit the form. But is there a better/cleaner way todo this?