Hello, discord.com is blocked in my country, for this reason I want to install a VPN that will work "only on discord.com" on my OpenWRT Xiaomi Router 4A Giga device. Other connections should continue as normal without VPN. Can you help me with this configuration, what should I do? Thanks.
You need policy based routing.
I have no idea how to do it though 
This should help:
https://openwrt.org/docs/guide-user/network/routing/pbr_netifd
If the site you want to reach has a known constant IP address you could just add a route to it via VPN in the regular routing table. Configure the VPN client so it does not (as is usually wanted) make itself the default route.
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