I guess the comment from Shaun was referring to a normal interface, not the failover interface.
The failover interface is not visible in the interface list neither via GUI or CLI, I also double check in 3600 unit.
The hardware failover only uses cable signal, there is no layer2/3 information, so no need to configure an IP.
You need a special cable, or just make one.
As you can see in this solution, you need a crossover cable with pins 7 and 8 changed:
https://support.f5.com/kb/en-us/solutions/public/1000/400/sol1426.html
There is also this solution that talks about a better cable:
https://support.f5.com/kb/en-us/solutions/public/15000/800/sol15802.html
This solution explains network failover and hardware failover:
https://support.f5.com/kb/en-us/solutions/public/2000/300/sol2397.html