Add-Type -Path "C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll"  
Add-Type -Path "C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"  
  
$siteURL = "https://contoso.sharepoint.com"  
$userId = "amos@contoso.onmicrosoft.com"  
$pwd = Read-Host -Prompt "Enter password" -AsSecureString  
$creds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($userId, $pwd)  
$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($siteURL)  
$ctx.credentials = $creds  

$authenticationCookies = $ctx.credentials.GetAuthenticationCookie($siteURL)
Write-Host $authenticationCookies