To get them linked, all you need to do is complete the verification on both accounts using the same identification document. After your accounts are approved, they will be automatically linked by the system.
Note: If your sub account has referral link applied to it, it will be removed once the accounts are linked
How do I know if my accounts are linked?
If you've already verified both accounts and you're not sure whether they have been linked, you can reach out to Support to check on it for you. Linked accounts are not visible through the site at the moment.