该问题可能是由于POST请求没有正确设置消息正文导致的。可以使用HttpClient类的PostAsync方法来发送POST请求，并通过StringContent类设置JSON消息正文。以下是一个示例代码：

using System.Net.Http;
using System.Text;
using System.Threading.Tasks;
using Newtonsoft.Json;

public class MyModel
{
    public string Name { get; set; }
    public int Number { get; set; }
}

public class MyController : ApiController
{
    [HttpPost]
    public async Task MyAction([FromBody] MyModel model)
    {
        // some code here
    }
}

public class MyTest
{
    [Fact]
    public async Task MyTestAction_ShouldReturnOk()
    {
        var client = new HttpClient();
        var url = "http://localhost:1234/api/MyController/MyAction";
        var model = new MyModel { Name = "test", Number = 123 };
        var json = JsonConvert.SerializeObject(model);
        var content = new StringContent(json, Encoding.UTF8, "application/json");
        var response = await client.PostAsync(url, content);
        response.EnsureSuccessStatusCode();
    }
}