If you do self-study you will need a training provider where you can sit your exams. First find one (https://www.aat.org.uk/training-providers/search) and make sure they accept external students. Then use that provider when you register.
Or just choose a provider and register. Later when you know exactly where you sit your exams write an email to AAT and ask them to change the provider in your AAT account.
Answers
If you do self-study you will need a training provider where you can sit your exams. First find one (https://www.aat.org.uk/training-providers/search) and make sure they accept external students. Then use that provider when you register.
Or just choose a provider and register. Later when you know exactly where you sit your exams write an email to AAT and ask them to change the provider in your AAT account.
Hope this helps.