Add X Forwaded For in HTTP monitor

Hi I want to add XFF in my HTTP monitor for LTM pool.

I have tried this and still marked as down :

GET /sys-info/healthcheck.json HTTP/1.1\r\nX-Forwarded-For: 172.6.172.14\r\nHost:\r\n\r\n

I use Big IP 12.00

Am i miss something in send string?

Thank you so much