Rewrite JSON value.

when HTTP_REQUEST {
  log local0. "starting collect"
      HTTP::collect [HTTP::header Content-Length]
      set clen [HTTP::header Content-Length]
}
when HTTP_REQUEST_DATA {
  binary scan [HTTP::payload] c* throwaway
  set incoming [HTTP::payload]
  log local0. "before: $incoming"
  regsub -all {,   "formName} $incoming {,  {"deviceName":"ESSApp"},  } newdata
  log local0. "new data is $newdata"
  HTTP::payload replace 0 $clen $newdata
  log local0. "after: [HTTP::payload]" 
}

$ curl -kv https://172.16.0.220/xxx/formservice -d '{"token":"044VZAliLSXEo2l4Xcv9DFPNLQKbX7p8E3i/Zt+ksskQ+E=MDE5MDA2MjgwOTQzNzQwNjUwMjI4NzM5OUVTU0FwcDE1OTgzNDA4NzQ3NDM=",",   "formName": "P59MBLVR_W59MBLVRE",   "returnControlIDs":"39|31|37"  }' -H 'Content-Type: application/json; charset=utf-8'
 
Aug 30 16:07:03 bigip1 info tmm[12235]: Rule /Common/json_modify <HTTP_REQUEST>: starting collect
Aug 30 16:07:03 bigip1 info tmm[12235]: Rule /Common/json_modify <HTTP_REQUEST_DATA>: before: {"token":"044VZAliLSXEo2l4Xcv9DFPNLQKbX7p8E3i/Zt+ksskQ+E=MDE5MDA2MjgwOTQzNzQwNjUwMjI4NzM5OUVTU0FwcDE1OTgzNDA4NzQ3NDM=",",   "formName": "P59MBLVR_W59MBLVRE",   "returnControlIDs":"39|31|37"  }
Aug 30 16:07:03 bigip1 info tmm[12235]: Rule /Common/json_modify <HTTP_REQUEST_DATA>: new data is {"token":"044VZAliLSXEo2l4Xcv9DFPNLQKbX7p8E3i/Zt+ksskQ+E=MDE5MDA2MjgwOTQzNzQwNjUwMjI4NzM5OUVTU0FwcDE1OTgzNDA4NzQ3NDM=",",  {"deviceName":"ESSApp"},  ": "P59MBLVR_W59MBLVRE",   "returnControlIDs":"39|31|37"  }
Aug 30 16:07:03 bigip1 info tmm[12235]: Rule /Common/json_modify <HTTP_REQUEST_DATA>: after: {"token":"044VZAliLSXEo2l4Xcv9DFPNLQKbX7p8E3i/Zt+ksskQ+E=MDE5MDA2MjgwOTQzNzQwNjUwMjI4NzM5OUVTU0FwcDE1OTgzNDA4NzQ3NDM=",",  {"deviceName":"ESSApp"},  ": "P59MBLVR_W59MBLVRE",   "returnControlIDs":"39|31|37"  }