Settings > System settings > General > Brand
Favicon (short for favorite icon) is the icon shown on the browser's address bar or next to the site name in the bookmark list. It will be displayed on the address bar when your affiliates sign up or log into their account.
A favicon helps your affiliate program to be more professional. To know how to Add a favicon to your Shopify store, you can follow this guide from Shopify.
To get your Favicon URL, go to your webpage and right-click > View page source.
Press Ctrl + F > type icon > you will see your favicon URL next to it.
Copy the URL > paste it into the Favicon URL box and add https: at the beginning of the link, just like this:
Finally, don't forget to click Save changes.