create subdomain, pointing to other domain

[Goddchen]

hi, i am 1und1-profiseller, 1und1 is a german isp.
i have homepage there but it's very cryptic like <long_id>.profiseller.de...
i have a root server with plesk installed, let's say my domain is domain.de. now i want to create "profiseller.domain.de" which should reference to the long and cryptic url mentioned above. how can i do that?

Goddchen