For starters, you need to specifically request the properties, as by default Get-MgUser returns only a small subset.
Get-MgUser -Property DisplayName,onPremisesExtensionAttributes,UserPrincipalName
Do note that you have to request each property you plan to use, including those used for filtering. Next, don't use Format-table, as it does not mix with Export-CSV. Use another select statement instead, which gives us:
Get-MgUser -Property DisplayName,onPremisesExtensionAttributes,UserPrincipalName | Where-Object {$_.UserPrincipalName -Like "*michev.onmicrosoft.com"}| Select-Object -ExpandProperty onPremisesExtensionAttributes | select DisplayName,UserPrincipalName,ExtensionAttribute* | Export-CSV -nti blabla.csv