Open Setup → Email Template and click on the mail where you want to add the Auto Login Link.
At the WHMCS Mail Editor you can add &{$modautologin} to existing links or create a new link with &{$modautologin} at the end.
Examples: {$whmcs_url}viewinvoice.php?id={$invoice_id}&{$modautologin}
https://your-whmcs.com/clientarea.php?{$modautologin}
https://your-whmcs.com/clientarea.php?{$modautologin}