HTTP Monitor : Send a dummy X-Forwarded-For header

Back end servers need the X-Forwarded-For header and I am trying to send a dummy X-Forwarded-For header in the monitor without any success.

Below is my "Send String" GET /TestApp/ping.txt HTTP/1.0\r\nX-Forwarded-For: 10.10.10.10\r\n

However, I am not able to get the X-Forwarded-For header in the back end server. Will appreciate the help.