C# HttpClient authorization header removed after send to server

Question

I wanna send request to external API through the HttpClient from my own API in c#. I am using dot net 5 and basic authentication. Here is my code:

var client = new HttpClient 
{
    BaseAddress = new Uri(baseUrl)
};

HttpRequestMessage requestMessage = new HttpRequestMessage(HttpMethod.Put, "apiUrl");

client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

var param = JsonConvert.SerializeObject(new
{
   param1="",
   param2=""
});

requestMessage.Content = new StringContent(param, Encoding.UTF8, "application/json");
requestMessage.Headers.Authorization = new AuthenticationHeaderValue("Basic",
     Convert.ToBase64String(Encoding.ASCII.GetBytes($"{user}:{pass}")));

HttpResponseMessage response = await client.SendAsync(requestMessage);

Usually, I send http request like this. but now I have some problem.

After line HttpResponseMessage response = await client.SendAsync(requestMessage); authorization header removed from my request header and I got UnAuthorized http error.

I know that when redirection occurs, authorization header removed for security reason. Also I'm not sure about redirect in external API.

I add the HttpClientHandler with AllowAutoRedirect = false to my HttpClient

var handler = new HttpClientHandler()
{
    AllowAutoRedirect = false,
};
var client = new HttpClient (handler)
{
    BaseAddress = new Uri(baseUrl)
};

Now I got Redirect error 301(Permanently moved).

I decided to test the code in Postman. By default, when I call API in Postman, I got http error 405 method not allowed and error detail like this:

{ "detail": "Method "GET" not allowed."}

External API method is PUT. but here I got GET Error. I tried many options in postman and finally I find the option in Postman:

When I toggle it on, external API work's properly. Also I test it with Insomnia and it's work properly.

Does it related to my code or dot net 5 or what other thing in my code or it's related to external API?

If it's related to my code, How can I solve the error?

If error related to external API, why Postman and Insomnia response is ok?

External API has Core Policy for specific domain and I send request from other domain. All I Know is that the CORS policy applied in browser. not in Postman, Insomnia or C# Code. What about CORS? Does it related to CORS? if Yes, what shall I do?

I will be grateful for your answer.

Update

I detect WWW-Authenticate: JWT realm="api" in the response header. What exactly is it? and what shall I do?

Answer 1

I find out the problem. It's really Ridiculous. When I use URL like www.abc.com/api/something the request gets 301 error and postman sends another request like www.abc.com/api/something/. The difference is just / at the end of request. I tried new URL in postman and first request got ok. Also I tried URL in my C# code and again its ok. But i could not understand why.

Thanks a lot dear @pharaz-fadaei