How to post incorrect byte characters such as curl

$echo-e'str=\xff'>invalid_bytes.txt
$ hexdump-Cinvalid_bytes.txt 
000000007374 723d ff0a | str=..|
00000006
$ curl-d@invalid_bytes.txt http://httpbin.org/post
{
  "args": {}, 
  "data":", 
  "files": {}, 
  "form": {
    "str": "\ufffd"
  }, 
  "headers": {
    "Accept": "*/*", 
    "Connection": "close", 
    "Content-Length": "6", 
    "Content-Type": "application/x-www-form-urlencoded", 
    "Host": "httpbin.org", 
    "User-Agent": "curl/7.52.1"
  }, 
  "json"—null, 
  "origin": "xxx.xxx.xxx.xxx", 
  "url": "http://httpbin.org/post"
}

$echo-e'str=\xff' | curl-d@-http://httpbin.org/post

$curl-d "$(echo-e'str=\xff')"http://httpbin.org/post

$DATA=$(echo-e'\xff') curl-d "str=${DATA}" http://httpbin.org/post