You can find your test code on your test request form, this is the form that you would have received in your test kit box. The test code will be made up of 7 letters and can be found on the right side of the form above the QR code. 

It's important to note that this is not the registration code, your registration code will be generated once you have registered your test kit in the H&B&Me app. It starts with LML- and you'll need to write this on the test request form before posting this off with your sample.