You can connect your X (Twitter) account to your fomo profile at any time. On the mobile app, open the Portfolio tab (its icon is your avatar), tap the gear icon to open Settings, then Profile and Account.
The Link X button sits under your login email. Once linked, it shows your X username, and tapping it lets you unlink. On the web app, open Settings, then Profile, and use Link X account. Linking also sets your display name and profile photo to the ones on your X account.
If you previously connected and then unlinked your X account, you can reconnect it using the same path as outlined above. You can only connect your X account to one fomo account at a time.
Your username while X is linked
While your X account is linked, your fomo username matches your X handle and can't be changed on its own. To use a different username, unlink X first, then set any available name.
Linking X is also how you claim your X handle as your fomo username. A handle that belongs to an established account on X is reserved for that account, and a handle another fomo user already holds can't be taken.