Hi Todd

See code sample below, I hope it helps. 

// Get Order reference 

var orderReference = _orderRepository.SaveAsPurchaseOrder(cart);

// Load purchase order 

IPurchaseOrder purchaseOrder = _orderRepository.Load<IPurchaseOrder>(orderReference.OrderGroupId);





// loop through all payments in the PurchaseOrder, find payment with id equal guidPaymentMethodId, set TransactionId
foreach (var payment in purchaseOrder.Forms.SelectMany(form => form.Payments).Where(payment => payment.PaymentMethodId.Equals(paymentMethodId)))
{
    payment.TransactionID = token;
    payment.ProviderTransactionID = token;
}