First you click on "Settings" please and then on "Own Domains":
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Then you see the menue for your own domains:
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Before you can add a new Subdomain it is mandatory that you add an imprint and privacy-disclaimer.
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First the imprint:
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Then the privacy-disclaimer:
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Now you can add your new subdomain:
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Once you have added your subdomain the system will generate a destination domain for you:
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You need this destination domain for your settings at your domainhost.