Couldn't add partner config override to PayPal Marketplace API

I would like to implement the market place with Paypal. I got as far as 4 Onboard sells before payment, but I can't add a return URI.

Partner Referrals API should work, but wouldn't work...

Error

Request is not well-formed, syntactically incorrect, or violates schema. The value of a field should be of type 'object.

uri = URI.parse("https://api-m.sandbox.paypal.com/v2/customer/partner-referrals")
request = Net::HTTP::Post.new(uri)
request.content_type = "application/json"
request["Authorization"] = "Bearer #{access_token}"
request.body = JSON.dump({
  "partner_config_override" => {
    "partner_logo_url" => "https://www.paypalobjects.com/webstatic/mktg/logo/pp_cc_mark_111x69.jpg",
    "return_url" => "https://xxxxxxx.com/callback",
  },
  "operations" => [
    {
      "operation" => "API_INTEGRATION",
      "api_integration_preference" => {
        "rest_api_integration" => {
          "integration_method" => "PAYPAL",
          "integration_type" => "THIRD_PARTY",
          "third_party_details" => {
            "features" => [
              "PAYMENT",
              "REFUND"
            ]
          }
        }
      }
    }
  ],
  "legal_consents" => [
    {
      "type" => "SHARE_DATA_CONSENT",
      "granted" => true
    }
  ],
  "products" => [
    "EXPRESS_CHECKOUT"
  ]
})
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