This function is not avaiable in all hosting plans. Please precheck the avability of this function in your plan.

To manage an external domain in your webspace contract you first need your subdomain to point to the main domain of your EUserv webspace contract via CNAME.  
An manual regarding this subject can be found here:

How can I set a CNAME entry for my subdomain to to the mainn domain of my webspace contract ?

Setting up the external subdomain in your webspace contract "EUserv":

1. Log in to the customer center

2. Choose your webspace contract:

3. Click on the menu entry "Subdomains"

4. Click on the "Anlegen" button next to "Neue Subdomain".

5.Enter the directory on your webspace to which your subdomain is supposed to be redirected under "Zielverzeichnis"

6. Choose "Subdomain von extern". Enter the external subdomain that is redirected to your webspace via CNAME.

7. Click on the "Speichern" button to apply your changes.