Navigate to [https://ads.google.com/](https://ads.google.com/)
Click on the Performance Max campaign you would like to apply the brand exclusion to.
Click on 'settings'.
Click on 'Additional settings'
Click on 'Brand exclusions'
Write your brand here.
If your brand isn't listed, click "New brand list"
Write your brand name and click the relevant listing and URL.
Click "Save"