To search your Office 365 users with particular initial characters
First connect to Microsoft Online Service
To Search for users whom their display names contain “Top” you can use the following powershell
get-msoluser -all | where-object {$_.displayname -like “top*”} | ft displayname,userprincipalname,proxyaddresses
Search for users whom their UPN contains “TOP” in the start
get-msoluser -all | where-object {$_.userprincipalname -like “top*”} | ft displayname,userprincipalname,proxyaddresses
Hope this helps