add the following to the frontend section of your haproxy configuration
## ------ LOG REQ BODY ----------
option http-buffer-request
declare capture request len 40000000
http-request capture req.body id 0
log-format "%ci:%cp [%tr] %ft %b/%s %TR/%Tw/%Tc/%Tr/%Ta %ST %B %CC %CS %tsc %ac/%fc/%bc/%sc/%rc %sq/%bq %hr %hs %{+Q}r %[capture.req.hdr(0)]"
## --------------------