To complete the federation configuration, you need to add a TXT record in DNS for the domain you want to use as the account ...

To complete the federation configuration, you need to add a TXT record in DNS for the domain you want to use as the account namespace and for any other domain you want to add as a federated domain.The format of the TXT record should be "example.com IN TXT hash-value" where example.com is replaced with the domain you want to configure for federation and hash-value is replaced with the proof value generated with "Get-FederatedDomainProof -DomainName example.com".After the TXT record is available in DNS, complete the federation configuration by using the Manage Federation Wizard in the Exchange Management Console or the Set-FederatedOrganizationIdentifier cmdlet in the Exchange Management Shell.