Dual WAN - Traffic of PPTP Client over specific WAN

Hi,

I have a Router with two WAN connections.
The traffic from LAN is passed either over WAN1 or WAN2, depending on the IP Address / Subnet.
Using Prerouting Rules I was able to “mark” the traffic from each device. Such that I am able to route it over the right WAN connection.

Now I also want to integrate the PrivateInternetAccess-VPN service to forward traffic of certain devices over VPN.
The integration can be done as described here: https://matthewmcclatchey.com/using-private-internet-accesss-vpn-with-mikrotiks-routeros-via-pptp

The problem is that my PPTP Client connection (to the VPN Server / PPTP Server) must be established over WAN2.
Meaning that all traffic must go over WAN2.
But I have no clue how to bind the PPTP Client connection to a certain WAN interface.

I know that it might be a stupid question, but I’m using RouterOs for the first time (was using PfSense before).
Therefore I am not experienced with such scenarios.

Can someone give me a hint?

Thank you in advance

Kind regards
Michael