You can provide your users with a url that automatically logs them in. To set this up, you need a pair of username => password in the urlKeys configuration setting.
<?php
return [
'urlKeys' => [
'steeve' => 'BepmD8GQBZpaFpXQ',
],
];
After setting that up, you can login by going to http://my-site.test/autologin?key=BepmD8GQBZpaFpXQ. This will login the user steeve automatically!
If you want to redirect to the control panel dashboard, add cp=true to the url: http://my-site.test/autologin?key=BepmD8GQBZpaFpXQ&cp=true