One question that comes up time and again in engagements is “Can I change the URL for SharePoint Online to match my domain name?” and the short answer is no.. whatever you made your tenancy name when you created it, is what will form the URL for SharePoint and OneDrive and other URL based services (E.g. PowerBI etc..)
So if you named your Tenancy FlibbleWorks.OnMicrosoft.Com, but really want your domain name of FlibbleFantastic.co.uk to be used as FlibbleFantastic.SharePoint.Com then make sure when you get to this screen in the sign-up process
that you put the correct tenancy name that you want to be using for SharePoint and other services going forward. Because even after you align your domain name of FlibbleFantastic.co.uk and your users are all signing in as username@flibblefantastic.co.uk, they’ll still be logging on to FlibbleWorks.sharepoint.com for ever more!!
Paul.
PS I realise I’m posting this on April 1st.. But this is NOT an April Fools! (See here for the Official Line from MS!)