您混合了多种发送数据的方式：

• 数据字符串包含 application / x-www-form-urlencoded data：

You're mixing up multiple ways of sending data:

• The data string contains application/x-www-form-urlencoded data:

?username=...&type=text

"?username=...&type=text"

"\"?username=...&type=text\""

using (var client = new HttpClient())
{
    response = await client.PostAsJsonAsync(BaseUrl, new
    {
        username,
        password,
        api_key = apikey,
        from,
        to,
        text,
        type = "text"
    });
}