ExO via client secret

$tenantID = "tenant.onmicrosoft.com" #your tenantID or tenant root domain
$appID = "12345678-1234-1234-1234-1234567890AB" #the GUID of your app
$client_secret = "XXXXXXXXXXXXXXXXXXXXXX" #client secret for the app

$body = @{
    client_id     = $AppId
    scope         = "https://outlook.office365.com/.default"
    client_secret = $client_secret
    grant_type    = "client_credentials"
}

$authenticationResult = Invoke-WebRequest -Method Post -Uri "https://login.microsoftonline.com/$tenantId/oauth2/v2.0/token" -ContentType "application/x-www-form-urlencoded" -Body $body -ErrorAction Stop

$token = ($authenticationResult.Content | ConvertFrom-Json).access_token
$Authorization = "Bearer {0}" -f $Token
$Password = ConvertTo-SecureString -AsPlainText $Authorization -Force
$Ctoken = New-Object System.Management.Automation.PSCredential -ArgumentList "OAuthUser@XXXXXXXXXXXXXXXXXX",$Password #replace your tenantGUID here
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri "https://outlook.office365.com/PowerShell-LiveId?BasicAuthToOAuthConversion=true&email=user@tenant.onmicrosoft.com" -Credential $Ctoken -Authentication Basic -AllowRedirection -Verbose
Import-PSSession $Session