In the product settings create 2 custom fields named Username and Password.
In default_values.php change these 2 lines:
$billing_api_values["user_name"] = $params["username"];
$billing_api_values["user_password"] = $params["password"];
to this:
$billing_api_values["user_name"] = "CustomField:Username";
$billing_api_values["user_password"] = "CustomField:Password";
This will only work for the user's first game server because integration is done with the billing id. For example if WHMCS sends client billing id 10 and there is already a user in TCAdmin with this billing id it will use that userid and password. It will not use the values that the client entered in the order form. To ask for the TCAdmin user and password only once use custom client fields.
