OpenVPN over IPSEC - forcing source IP or interface

Hello,
I would like to make an OpenVPN connection to another Mikrotik device, with IPSec in the middle. (the OpenVPN server should not be reached other than IPsec)

Setup like the following:
DeviceA(RB750)-Internet via PPPoE
DeviceB(RB450G) - Internet via Leased Line
DeviceC(RB450G) - uplink to DeviceC via microwave

I set up the IPSec between DeviceA and DeviceB, and i can ping DeviceC from DeviceA - if i set the correct source IP in ping or traceroute
From DeviceC i can ping DeviceA as well, so IPSec works as expectect.

As i mentioned, i would like to setup an OpenVPN tunnel between DeviceA and DeviceC to reach the LAN behind DeviceC.

The problem is, when i would like to initiate OpenVPN tunnel, the source of package is bound to Internet interface (so it’s picked from the route table).
I’ve tried to do firewall mangle to mark route and set preferred outbound IP to packet, but does not helped. (or probably i’m failed at that point)

I would be pleased to have any idea to properly set the OpenVPN client to right IP or interface! (OpenVPN would be used in routing mode)