HttpClient send XML PostAsync Malformed request: Premature end of file?

Question

I am trying to send a xml to a server and it always throw an exception Malformed request: Premature end of file.. . Using the Postman it works well but using HttpClient it doesn't, I think the problem is with headers what server needs is Content-Type", "application/xml and I cannot do it works

How could I fix it?

trying

//HttpClient
            HttpClient hClient = new HttpClient();            
            hClient.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/xml");
            
            //post            
            var contentString = new StringContent(xml, Encoding.UTF8, "application/xml");
            HttpResponseMessage response = hClient.PostAsync(URL_FINAL, contentString).GetAwaiter().GetResult();
            var resultContent = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();

xml

<?xml version="1.0" encoding="utf-16"?>
<payment xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <mode>default</mode>
   <method>creditCard</method>
   <sender>
      <name>Meu Nome</name>
      <email>email@gmail.com</email>
      <phone>
         <areaCode>17</areaCode>
         <number>9999999999</number>
      </phone>
      <documents>
         <document>
            <type>CPF</type>
            <value>9999999999</value>
         </document>
      </documents>
      <hash>5e5240axxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxea8a</hash>
   </sender>
   <currency>BRL</currency>
   <notificationURL>https://dominio.com/StatusPagamentoTransaction</notificationURL>
   <items>
      <item>
         <id>2</id>
         <description>produto</description>
         <quantity>2</quantity>
         <amount>2.00</amount>
      </item>
   </items>
   <extraAmount>0.00</extraAmount>
   <reference>R748</reference>
   <shipping>
      <addressRequired>false</addressRequired>
   </shipping>
   <creditCard>
      <token>91999999999999999999b0f</token>
      <installment>
         <quantity>1</quantity>
         <value>2.00</value>
      </installment>
      <holder>
         <name>nome proprietario cartao</name>
         <documents>
            <document>
               <type>CPF</type>
               <value>99999999999</value>
            </document>
         </documents>
         <birthDate>18/12/1964</birthDate>
         <phone>
            <areaCode>17</areaCode>
            <number>99999999</number>
         </phone>
      </holder>
      <billingAddress>
         <street>rua A</street>
         <number>3333</number>
         <district>bairro</district>
         <city>cidade</city>
         <state>SP</state>
         <country>BRA</country>
         <postalCode>05407002</postalCode>
      </billingAddress>
   </creditCard>
</payment>

Exception

<?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?><errors><error><code>11213</code><message>Malformed request: Premature end of file..</message></error></errors>

Answer 1

Try to set Accept header

hClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));

Instead of

hClient.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/xml");