var client = new RestClient("https://localhost:44380/api/broker/Autenticacaoestabelecimento/login");

var request = new RestRequest(Method.POST);

request.AddHeader("Content-Type", "application/json");

request.AddParameter("application/json", "{\n\t\"UserName\" : \"caio.fontes\",\n\t\"Password\": \"123456\"\n}",  ParameterType.RequestBody);

IRestResponse response = client.Execute(request);

Console.WriteLine(response.Content);

<?php

$curl = curl_init();

curl_setopt_array($curl, array(
  CURLOPT_URL => "https://localhost:44380/api/broker/Autenticacaoestabelecimento/login",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 0,
  CURLOPT_FOLLOWLOCATION => true,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS =>"{\n\t\"UserName\" : \"caio.fontes\",\n\t\"Password\": \"123456\"\n}",
  CURLOPT_HTTPHEADER => array(
    "Content-Type: application/json"
  ),
));

$response = curl_exec($curl);

curl_close($curl);
echo $response;