我有一个最小的API服务与MapPost：

app.MapPost("/sendmessage", (RabbitMessage rm) => server.SendMessage(rm.exchange, 
rm.routingkey, rm.message));

app.Run();

record RabbitMessage(string exchange, string routingkey, string message);

它在使用Postman发送JSON时工作正常：

{
    "message": "msg",
    "routingkey": "freestorage",
    "exchange": "system"
}

但是从C#客户端：

var kv = new Dictionary<string, string> {
    { "exchange", "system" },
    { "routingkey", routingkey },
    { "message", message }
};

var content = new FormUrlEncodedContent(kv);

string contentType = "application/json";

if (Client.DefaultRequestHeaders.Accept.FirstOrDefault(hdr => hdr.MediaType == contentType) == default)
    Client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue(contentType));

var response = await Client.PostAsync("sendmessage", content);

响应是UnsupportedMediaType。将这些值发送到最小API的正确方法是什么？或者我需要对API本身进行不同的设置吗？