When you connect your domain and encounter This domain was added by another HubSpot customer, it likely means that the subdomain you're trying to connect has already been added to a HubSpot account.
For example, if you already connected your subdomain in a separate trial HubSpot account from your paid HubSpot account, this error will appear.
To resolve this error, access the account with the connected domain and remove it. This should allow you to connect the same domain to your current account. If you're unable to access that account or you believe another customer account is using the domain in error, reach out to HubSpot Support.
Domains & URLs
Table of contents/cos-general/error-the-domain-was-added-by-another-hubspot-customer