F5-ASM

I think what you are looking is to consider the IP inside the X-FF header the client IP. In order to make the WAF to trust that header you need to modify the ASM policy to "trust" x-forwarded-for header. You can enable this feature in the Configuration utility by selecting the Trust XFF Header check box in the security policy properties advanced configuration settings.
Overview of the Trusted X-Forwarded-For header (f5.com)