Howto create an inbound NAT rule to a dynamic WAN (Self) address?

[willink2]

For practical reasons I want to create a NAT rule to allow external connections to the WAN interface on port 123 and reflect these to the Wireguard service on a default port 51820 on the firewall it self (So form WAN:123 to WAN:51820). I need both options available externally, I don´t want to change the default Wireguard port.

I tried to create a NAT rule. In the ¨Redirect Target IP"  box I can either select, "Single host or network" or Alias. I cannot select WAN interface, which is problematic because I have a dynamic WAN address. This  means both option won´t work...

I come from PFSense an here I could select "WAN Address" which would always be my current WAN IP address.

What is the logic behind this and how can I create a NAT rule to the firewall it self?