M-Files and windows authentication from Web API

Hi community and M-Files, 

I am tasked to design new Web API which should download and open documents from M-Files. Our users are not satisfied with links created by M-Files.

I am struggling to authenticate user with SSO M-Files functionality. Basically, I have .NET 6 Web Api running under own app pool. It tries to authenticate over SSO functionality:

public async Task AuthenticateUsingSSO(string vaultGuid)
		{
			// Build a request to WebServiceSSO.aspx.
			var request = new RestRequest($"/WebServiceSSO.aspx?popup=1&vault={vaultGuid}");

			try
			{
				// Execute the request.
				var response = await _client.GetAsync(request);

				if (response.StatusCode == HttpStatusCode.Unauthorized)
					throw new AuthenticationException(response.Content, response.ErrorMessage);

				// Populate our cookie container with the cookies (i.e. session tokens)
				// returned by the request to WebServiceSSO.aspx.
				if (response.Cookies != null)
				{
					foreach (var cookie in response.Cookies.AsEnumerable())
					{
						_client.CookieContainer.Add(new Cookie(cookie.Name, cookie.Value, cookie.Path, cookie.Domain));
					}
				}
			} 
			catch (Exception ex)
            {
				throw new AuthenticationException(ex);
            }
		}

When I execute "await _client.GetAsync(request);", I already starts getting 401 Unauthorized error. 

RestClient is initialized like this:

return new RestClient(
				new RestClientOptions
				{
					Credentials = CredentialCache.DefaultNetworkCredentials,
					BaseUrl = new Uri(_baseUrl)
				}
			);

This should pick up credentials from executing user.

I am following examples from M-Files client library from Git and I am using RestSharp library. 

Has anyone implemented windows authentication / SSO in IIS? Basically taking user credentials with windows authentication and logging with those credentials over SSO API from M-Files.

I would appreciate any sharing in this area. It would be also great to know if someone used .NET Core or .NET 6 to implement this one.

Best regards,

Dejan