Hi @Trevor Westin ,
Thanks for reaching out.
The document you are referring is of Entra ID which has different portal than B2C tenant due to which you are not able to find custom domain in Azure AD B2C portal.
You will find that custom domain option in your Microsoft Entra ID(workspace tenant) than Azure AD B2C(customer tenant).
To configure custom domain in Azure AD B2C, refer https://learn.microsoft.com/en-us/azure/active-directory-b2c/custom-domain?pivots=b2c-user-flow
Hope this will help.
Thanks,
Shweta
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