There's no default-domain setting to configure. When you connect more than one domain, the domain field on new links, Bio Pages, and Smart Picks simply remembers whichever domain you picked most recently, as a convenience - it isn't a setting you turn on, and there's no way to force a specific domain to always be pre-selected.

golo.to is the only true fallback: any domain you haven't connected, or a domain you haven't picked, defaults to it.

Adding a custom domain - Type it into Settings - then complete DNS setup and verification.
Choosing a custom slug - Slugs are unique per domain, not shared across all of them.
Connecting a custom domain - Add your domain and verify it with DNS records.
Creating a branded link - Pick your verified domain when creating the link.

Setting a default domain

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