I am not sure if it makes sense to install a web- and mailserver control panel on a system where you do not have administrative permissions. You will need root access to manage the installation. As an alternative, your IT department should install Plesk for you and give you the admin link. However, then they can equally well grant root access, because with the admin login you can use the SSH extension, which again gives you root access. It really does not make much sense to be admin of an installation yet to not be administrator of the underlying server.