You can add a custom domain at any time!
No plan is required to purchase a custom domain through Universe as a custom domain is a separate annual subscription. If you wish to connect an external domain that you've already purchased through another registrar (i.e. Namecheap, GoDaddy, etc.) a subscription to the Base, Pro or Complete plan is required.
To purchase a custom domain when creating a new site
When prompted to "Choose a new domain" search for a custom domain
Review available domain names
Tap the price of the respective domain to complete your purchase (Note: the price of domains varies by TLD)
To purchase a custom domain on an existing site:
Choose the site you want the domain to be attached to from Your Sites
Navigate to Settings
Select Domain
Search for a custom domain
Select the domain you wish
To connect a domain you already own when creating a new site
Scroll to the bottom and tap Use a domain you already own
Upgrade to a Base, Pro or Complete Plan
Select your specific host/registrar and follow the further instructions on the screen
Ask the Community
Do not worry. You are not alone! Ask other Universe creators how they solved this problem. Visit the community. ✌️