To authorize a brand to an Amazon buyer account, follow these steps:

1. Log in to the Amazon seller backend and select "Brand Flagship Store" in the menu bar.

2. On the "Brand Flagship Store" page, select "Brand Registry Support".

3. On the "Brand Registry Support" page, select "Contact Brand Customer Service".

4. On the "Brand Customer Service" page, fill in the relevant information and submit the work order.

5. Wait for a reply from Amazon customer service and provide supporting documents such as a brand authorization letter.

6. Brand authorization can be completed after Amazon's review is passed.

It should be noted that brand authorization requires the provision of valid brand authorization letters and other supporting documents, and the information provided must be true, accurate and complete. In addition, the authorized brand needs to exist on Amazon and comply with relevant regulations and requirements.