% curl \
    -XPOST https://httpbin.org/post \
    -G \
    --data-urlencode foo=bar \
    --data-urlencode baz=quux \
    -T <(echo -n '{"spam": "eggs"}')

{
  "args": {
    "baz": "quux",
    "foo": "bar"
  },
  "data": "{\"spam\": \"eggs\"}",
  "files": {},
  "form": {},
  "headers": {
    "Accept": "*/*",
    "Host": "httpbin.org",
  },
  "json": {
    "spam": "eggs"
  },
  "url": "https://httpbin.org/post?foo=bar&baz=quux"
}