How to select values from curly brackets?

PowerShell Help
1

The command Get-AzureADAuditDirectoryLogs -Filter “initiatedBy/app/displayName eq ‘Skype For Business Powershell Server Application’” | select targetresources, activitydatetime | FL gives me this output

TargetResources : {class TargetResource {
Id: bfa9ddfa-e22e-4227-914b-f73781f22026
DisplayName:
Type: User
UserPrincipalName: John.Smith@company.com
GroupType:
ModifiedProperties: class ModifiedProperties {
}

}
}
ActivityDateTime : 9. 2. 2020 17:46:34

How can I select only userPrincipalName from this class? Thanks

2 
(Get-AzureADAuditDirectoryLogs -Filter "initiatedBy/app/displayName eq 'Skype For Business Powershell Server Application'" | 
select targetresources).UserPrincipalName
3

thanks but when I use this command the output is empty

PS C:\WINDOWS\system32> (Get-AzureADAuditDirectoryLogs -Filter “initiatedBy/app/displayName eq ‘Skype For Business Powershell Server Application’” |
select targetresources).UserPrincipalName

PS C:\WINDOWS\system32>