Your Drone Tweaks credentials consist of your order number and the first 8 characters of your email, up to the "@".
Username - getting the order number
Your order number can be found in the email you received right after ordering. The subject will be something like "Thanks for shopping with us (#11272)", where 11272 would be your order number. See the image below for an example.
Please make sure to also check your spam folder if you can't find the email.