Though the Hetzner wiki 1, 2 gives good info on how to configure your additional ip on the server, one important info is worth mentioning is:
Hetzner assigns a MAC address to the newly assigned IP. In the bridge mode, all you have to do is, assign this MAC address manually to your VM OS and set ip to be assigned using DHCP. A simple restart of the network service will get you the new ip assigned and your guest os is ready to be reachable from the outside world.