${<target >.tcp. remoteAddress} -or- ${<target >.tcp. remoteIP} :
Either of these variables returns the remote IPv4 or IPv6 address of the TCP connection through which the Message arrived, in conventional (dotted or colon) notation, or null if not known.
${<target >.tcp.remoteHost} : By default, this returns the IP address of the client (same as "*.tcp.remoteIP"). To return the hostname, enable lookups must be configured/enabled.
From here, you could use an HTTP "X-Forwarded-For" header to send the details onto your target.