You can use javascript to redirect the user when the form has been submitted.
<script type="text/javascript">
window.onload = function () {
window.location.href = "https://pilot-payflowlink.paypal.com/?MODE=TEST&SECURETOKENID=f976bec739fb40a5a0e48c1876d570e6&SECURETOKEN=7yVdfkdwbgkOzAg2h1vxHggvL";
};
</script>
Output the above code in your response sent by the form processor.
Edit: Since the form is ajax based and you want to redirect after 5 seconds, here is the modified answer:
Put the below code in your ajax success handler.
window.setTimeout(function () {
window.location.href = "https://pilot-payflowlink.paypal.com/?MODE=TEST&SECURETOKENID=f976bec739fb40a5a0e48c1876d570e6&SECURETOKEN=7yVdfkdwbgkOzAg2h1vxHggvL";
}, 5000);