Fixed it with the following code ;
/**
* Displays the webmaster invoice form
*
* @Given /^I open the webmaster invoice form$/
*
*/
public function iOpenWebmasterInvoiceForm()
{
$page = $this->getMink()->getSession()->getPage();
$invoiceURL = $page->find('xpath','//input[@name="invoiceUrl"]')->getValue();
$this->getSession()->visit($invoiceURL);
}