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Arcadier marketplaces allow you to use either your own email/username and password or your Google or Facebook account to login to your marketplace.
To enter your admin portal, simply go to https://yourmarketplacename.arcadier.io/admin and login with your account. This will still apply even if you have a custom domain. Do note that you should be logging into your admin portal on web, and not mobile, as it has been optimised for web only.
Alternatively, you can login through through this link.