using Refit;
using System.Diagnostics;
using System.Net;

var refitsettings = new RefitSettings()
{
    CollectionFormat = CollectionFormat.Csv,
};
var service = RestService.For<ITest>(new HttpClient(new DebugHttpHandler()) { BaseAddress = new Uri("https://localhost") }, refitsettings);

await service.Foo(new TestObject("Test", "12345")).ConfigureAwait(false);

public interface ITest
{
    [Post("/foo")]
    Task<string> Foo([Body(BodySerializationMethod.UrlEncoded)] TestObject request);
}

public record TestObject(string Foo, string Bar);

public class DebugHttpHandler : HttpClientHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        Debug.WriteLine(await request.Content!.ReadAsStringAsync(cancellationToken).ConfigureAwait(false));

        return new HttpResponseMessage(HttpStatusCode.NoContent);
    }
}