MAC address of Floating IP address /Self IP address

If you have mac-masquerade configured, it will use the mac assigned to the floating self-IP's traffic group: tmsh list cm traffic-group traffic-group-1 mac

Otherwise, it should take on the mac-address associated with its vlan:

tmsh show net self (name of floating-ip)

If neither of those work for you, go to a device in the same subnet as the floating self-IP (such as the standby F5 if you have an HA pair), ping the floating IP, and look at your ARP table. F5 commands for those tasks:

tmsh run util ping (floating-ip)

tmsh show net arp | grep (floating-ip)

SOL14513: MAC address assignment for interfaces, trunks, and VLANs (11.x) - https://support.f5.com/kb/en-us/solutions/public/14000/500/sol14513.html