Custom Gateway Offsite Redirct

[jeffreyfritz333]

Using the samples and resources from 'How to build a Gateway Addon for GiveWP' there is a function called CreatePayment that is being hit:
public function createPayment(Donation $donation, $gatewayData)
At the end of this function is where it will return a redirect to the offsite payment gate, example:
return new RedirectOffsite($myUrl);

This RedirectOffsite is posting to $myUrl as a GET. How do i make this happen as a POST and include my form-data? My payment gateway url I am posting to is requiring I send it a POST with form data.