To enable the audit log via PowerShell, follow these steps:

1. Connect to Exchange Online PowerShell:

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential (Get-Credential) -Authentication Basic -AllowRedirection

Import-PSSession $Session

2. Once connected to Exchange Online, run the following commands:

Enable-OrganizationCustomization

Set-AdminAuditLogConfig -UnifiedAuditLogIngestionEnabled $true

The audit log will be enabled, but it may take some time before events start appearing.