Вытащил солюшен на уровень выше, чтобы прощё было дотнетить

VisaApiClient/ClientPartialMethods.cs

@@ -0,0 +1,31 @@
+using System.Text;
+
+// ReSharper disable UnusedParameter.Local
+
+namespace VisaApiClient;
+
+public partial class Client
+{
+    public AuthToken? AuthToken { get; set; }
+
+    private Task<HttpRequestMessage> CreateHttpRequestMessageAsync(CancellationToken cancellationToken)
+    {
+            var msg = new HttpRequestMessage();
+
+            msg.Headers.Authorization = new("Bearer", AuthToken?.Token);
+
+            return Task.FromResult(msg);
+        }
+
+
+    private async Task PrepareRequestAsync(HttpClient client, HttpRequestMessage request, string url, CancellationToken cancellationToken)
+        => await Task.CompletedTask;
+
+    private async Task PrepareRequestAsync(HttpClient client, HttpRequestMessage request, StringBuilder urlBuilder, CancellationToken cancellationToken)
+    {
+            await Task.CompletedTask;
+        }
+
+    private async Task ProcessResponseAsync(HttpClient client, HttpResponseMessage response, CancellationToken cancellationToken)
+        => await Task.CompletedTask;
+}

VisaApiClient/IClientBase.cs

@@ -0,0 +1,7 @@
+namespace VisaApiClient
+{
+    public interface IClientBase
+    {
+        AuthToken? AuthToken { get; set; }
+    }
+}

VisaApiClient/VisaApiClient.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net8.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+    <ItemGroup>
+      <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
+    </ItemGroup>
+
+</Project>