How to create a L2TP VPN tunnel from a particular WAN interface

I want to create an L2TP tunnel from my OpenWrt router to an L2TP VPN server. I want to bind the tunnel to a particular WAN interface.

I have successfully created an L2TP VPN tunnel but unable to bind it to a WAN interface. The L2TP session was created by that WAN interface which has the default route in the routing table.

Please give me a solution. Thanks in advance.

Add a route to the VPN server via the respective WAN gateway.
If the server is a domain name, you can use the VPN-PBR app.

The LAN IP of the VPN server is same then how to do equal metric routing for the LAN IP towards VPN tunnel in OpenWrt?
My end goal is to do load balancing on tunnel via mwan3.