You can create Restricted Access Code for clients to preview their app before publishing it. You will have to ask them to install the previewer app to their device. Then give them your Username to the platform together with the Restricted Access Code you have created for them.
Go to the application dashboard of the app you wanted to create a restricted access for your client. In the dashboard click "Application Settings"
Select Access Control (Privacy). Check the Allow restricted access checkbox to reveal the field to enter Access Code, Enter a code your client need for access. Click Save Changes button then Done.