Logging body return empty data

we are temporary are trying to log body respond to debug some issue
however setting the following in the front-end part of haproxy(HA-Proxy version 2.0.29-0ubuntu1.3):

option http-buffer-request
    declare capture request len 40000000
    http-request capture req.body id 0
    capture request header user-agent len 150
    capture request header Host len 15
    log-format '{"srcIP":"%[src]","backend":"%s","bIP":"%si","bPORT":"%sp","method":"%[capture.req.method]","user-agent":"%[capture.req.hdr(1),json(utf8s)]","uri":"%[capture.req.uri]","body":"%[capture.req.hdr(0)]"}'

return “body”:“-” , any idea how to get the actual body in the log?