Mult-WAN/Routing to Specific External Host

[bagofbones]

I feel like I'm missing something basic here, so I apologize now....

I have a Multi-wan setup on OPNSense, which is working well. I have a remote system that one of my clients uses wireguard to VPN into.  Wireguard Client running on Linux Client, Wireguard server running on a host external to my network. So, the flow looks like this:

Linux Client ------ OPNSense ------ External Host

 I'd like to setup a rule for any traffic to this remote host be pinned to one gateway. I setup a rule on that LAN network that says any traffic destined for that public IP uses this gateway, and that seems to work, however, once the wireguard connection is made, the traffic moves back to the main gateway. Is there a ways to accomplish what I'm trying to do with wireguard in the mix?