您只是传递字面硬编码值username"和password"，而不是变量的内容.

You're just passing the literal hard-coded values "username" and "password", not the contents of the variables.

尝试将变量连接到 URL 中:

Try concatenating the variables into the URL instead:

WebRequest request = WebRequest.Create("https://mydomain.com.au/LoginVerification.php?username=" + username + "&password=" + password + "&Token=" + Token);