Hetzner and failover IP

Hi.
I installed a Proxmox host on an Hetzner server, and I succesfully configured the additional IP addresses on a bridged network interface using the virtual MAC address given by Hetzner.
I also bought a failover IP which is a single IP address with netmask 255.255.255.255 and it cannot have a virtual MAC address because it's routed on the main IP address of the physical server, and not on the additional one.
I tried to get the failover IP working on the virtual machine, but without any success.

Could you help me please?

Thank you very much!
Bye.