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Select the Custom domains tab, For the domain you want to set up, select More Actions () > Show details.
Copy all of the DNS records, and add them to your DNS provider.
To add the DNS records to your DNS provider: (Ex: CloudFlare)
Step 3: Activate your domain
Select Products > Product URLs. Select the Custom domains tab. For the domain you want to activate select More Actions () > Activate domain.
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See Also
Ref
https://support.atlassian.com/organization-administration/docs/add-a-custom-domain/
https://support.atlassian.com/organization-administration/docs/troubleshoot-custom-domains/
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