If you need to know if a tenant name is available or taken in Office 365, I recommend to follow you one of the following approaches:
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First one is by using the following web page: http://o365.rocks/. There you will be able to do a quick check for the availability of a tenant name in Office 365.
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The other approach is just to use the PowerShell script you can find here: https://www.linkedin.com/pulse/how-check-office-365-tenant-name-availability-aaron-dinnage