By default, Social Snowball uses your primary domain in Shopify to generate unique affiliate links. To customize the subdomain of affiliate links, you must change your primary domain in Shopify.
Important note:
Changing the subdomain will not only apply to new affiliate links but will also update all existing affiliate links in your account. Your existing affiliates with links may receive an email regarding this update, depending on the status of the Affiliate program changed email as configured in the Customize > Emails tab of your dashboard.