Please perform the following instructions to create a PTR record:
1. Log in to your customer account on http://www.inwx.de.
2. Choose the menu item 'Nameserver' on the left navigation bar.
3. Click the eye-symbol right to your domain.
4. Click on 'add dns record'
5. As 'name' you can set the PTR name (e.g.: 18.104.22.168.in-addr.arpa), as 'type' you can set 'PTR', as 'value' you can set your hostname (e.g.: test1.example.com)
6. Now click 'save' to apply your changes.
You will find numerous help texts for each menu item by clicking on one of the question mark icons. In any case of doubt this will always help you.