A question about project's website customization

[vicky]

During the lists customization, I set a field as reference to "current
fields", for example user info , as release people of the notification.

What's strange is that my custom filed can not set default value as the
current user, but the similar fields set by system can, for example user
info as owner of problem.

How could I set the defalt value as the current user?

Thanks a lot.

Vicky