RestClient: Can't add custom audience via API on Facebook – Facebook api

I’m trying to add custom audience on Facebook using Ruby restclient gem but there is something with the payload param that’s not working.
I’m setting “multipart” param to the payload for RestClient but this doesn’t work neither. I get always a RestClient::BadRequest: 400 Bad Request

Here is an example of what I’m doing , any guest?

payload =  {:payload => 
   {:schema=>["FN", "LN", "EMAIL"],
    :data=>[
     ["31c88cbf2ceeb43fce68f027c10dd71b053672ce771fa5e4fc22422e633c6f11",
      "3565d65adb5cc2043e09097ee55ad430333a2a9a08a3a2b82c158b160ff12454",
      "d66746d2a1a6d488601e5603b8e9540c03e7e8063ae8aa921a3860007f478761"],
     ["3f03df52a32aafde65e6ee94b351970e0fa1481337b6b02f5f607d3357c1c368",
      "2ef9487065c3616d7c108c4b1c81e1b3afeee48f0373f7d5f5220bbc38db8ef5",
      "e4e91c99e2ae79c614e35de6cb429c32510dc2ecbf11c2a6e45a5872693542e6"],
     ["2de75eeda225156c96688715f0c2f86276dc0f10453dad716b9419729c46c5b8",
      "68c91edc2305b00a20a7febc616d2925e58bdbdf27fdcef8f84a1d1d8f37b3af",
      "a63ef4bd9dd5985f46674b4fb7f98b63c7144aae9b243281d01a66ad7659247e"]]
    },
   :access_token => "<ACCESS_TOKEN>",
   :multipart => true }
url = "https://graph.facebook.com/v2.7/<AUDIENCE_ID>/users"   

request = RestClient::Request.new(
:method => :post,
:url => url,
:payload=>payload)
response = request.execute

Building the request like this:

@payload="--474291rnContent-Disposition: form-data; name="payload[schema][]"rnrnFNrn--474291rnContent-Disposition: form-data; name="payload[schema][]"rnrnLNrn--474291rnContent-Disposition: form-data; name="payload[schema][]"rnrnEMAILrn--474291rnContent-Disposition: form-data; name="payload[data][]"rnrn31c88cbf2ceeb43fce68f027c10dd71b053672ce771fa5e4fc22422e633c6f11rn--474291rnContent-Disposition: form-data; name="payload[data][]"rnrn3565d65adb5cc2043e09097ee55ad430333a2a9a08a3a2b82c158b160ff12454rn--474291rnContent-Disposition: form-data; name="payload[data][]"rnrnd66746d2a1a6d488601e5603b8e9540c03e7e8063ae8aa921a3860007f478761rn--474291rnContent-Disposition: form-data; name="payload[data][]"rnrn3f03df52a32aafde65e6ee94b351970e0fa1481337b6b02f5f607d3357c1c368rn--474291rnContent-Disposition: form-data; name="payload[data][]"rnrn2ef9487065c3616d7c108c4b1c81e1b3afeee48f0373f7d5f5220bbc38db8ef5rn--474291rnContent-Disposition: form-data; name="payload[data][]"rnrne4e91c99e2ae79c614e35de6cb429c32510dc2ecbf11c2a6e45a5872693542e6rn--474291rnContent-Disposition: form-data; name="payload[data][]"rnrn2de75eeda225156c96688715f0c2f86276dc0f10453dad716b9419729c46c5b8rn--474291rnContent-Disposition: form-data; name="payload[data][]"rnrn68c91edc2305b00a20a7febc616d2925e58bdbdf27fdcef8f84a1d1d8f37b3afrn--474291rnContent-Disposition: form-data; name="payload[data][]"rnrna63ef4bd9dd5985f46674b4fb7f98b63c7144aae9b243281d01a66ad7659247ern--474291rnContent-Disposition: form-data; name="access_token"rnrnXXXXXXXXXXXXXXXXXXXXrn--474291--rn"

CURL Example that works:

curl 
  -F 'payload={ 
    "schema": [ 
      "FN", 
      "LN", 
      "EMAIL" 
    ], 
    "data": [["31c88cbf2ceeb43fce68f027c10dd71b053672ce771fa5e4fc22422e633c6f11",
      "3565d65adb5cc2043e09097ee55ad430333a2a9a08a3a2b82c158b160ff12454",
      "d66746d2a1a6d488601e5603b8e9540c03e7e8063ae8aa921a3860007f478761"],
     ["3f03df52a32aafde65e6ee94b351970e0fa1481337b6b02f5f607d3357c1c368",
      "2ef9487065c3616d7c108c4b1c81e1b3afeee48f0373f7d5f5220bbc38db8ef5",
      "e4e91c99e2ae79c614e35de6cb429c32510dc2ecbf11c2a6e45a5872693542e6"],
     ["2de75eeda225156c96688715f0c2f86276dc0f10453dad716b9419729c46c5b8",
      "68c91edc2305b00a20a7febc616d2925e58bdbdf27fdcef8f84a1d1d8f37b3af",
      "a63ef4bd9dd5985f46674b4fb7f98b63c7144aae9b243281d01a66ad7659247e"]] 
  }' 
  -F 'access_token=XXXXX' 
  https://graph.facebook.com/v2.7/XXXX/users --trace-ascii /dev/stdout