How to do Persistance Based on XFF IP Address

Hi, I have an issues where traffic destined for my F5 is passing through a cloud based Silverline and being NAT'd, therefore I am unable to do source-address persistance as the source IP is always that of the Silverline.

The Cloud Based Silverline is sending the real source IP in the XFF header and therefore I was wanting to know how I can do persiatcne based on that attribute which would then result in more efficient distribution of sessions.

Many Thanks

Danny