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I understand that you want to add a TXT record and verify your domain.
After you create the TXT record ,sometimes it may take up to 72 hours for the record that you created to propagate through the DNS system.
The following two links provide detailed information to add and verify domain.
If you are using Plan P, you can refer to this link for help.
And this link provide helpful information for Plan E users.
If there is other problem, you can feel free to post on this forum.