Option 1: Use Interfolio if you want to get the letter now. That allows you to collect confidential letters and they’ll submit it to AADSAS for you come May. There’s a fee but it’s not terrible and was worth the peace of mind for me having the letters in advance.
Option 2: Once AADSAS opens up, add them as a letter writer and they’ll receive an email with instructions on how to submit the letter.
You can’t upload a copy of the letter. Not saying you will, but then what would stop another applicant from forging a letter?