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