Configuring VPN on specific WiFi

I configured OpenVPN on openwrt and now all traffic network passes through the VPN. I want to pass the traffic of an specific WiFi through VPN and rest of them without VPN. As I looked up, I know I can use vpn-policy-routing to do it. but I couldn't find the way I should do this.
If you can help me with sharing your ideas, it would be great. thank you so much.