So you base64-encoded "username:password" and appended the result after "Basic ", as explained on the page I linked?
Edit: seems to work.
PHP Code:
public void OnPluginStart()
{
HTTPClient client = new HTTPClient("https://jigsaw.w3.org/HTTP");
client.SetHeader("Authorization", "Basic Z3Vlc3Q6Z3Vlc3Q=");
client.Get("Basic", OnHTTPResponse);
}
public void OnHTTPResponse(HTTPResponse response, any value)
{
PrintToServer("[Basic] %d", response.Status);
}
This returns a 200 response. If you comment out client.SetHeader() it returns a 401 response.