Default value for custom fields in New Issue

Hello,

In our Eventum setup, we created several custom fields (date type, drop
down, string).

When user create a new issue, we would like to populate those fields with a
default value based on certain criteria. Example: user's region based on
user's email-id, date = system's date, etc.

How can it be done?
Please advice. Thank you for your help and attention.

Sincerely,
Lukeman