How to add a field linking a doctype's field?

Name, Email

Username - Link - User
UserEmail ? → how to link to user’s email?

From doctype, in UserEmail field set option as

Nice! Field type is ? @saurabh6790 Same as UserEmail?

After many many tries, I haven’t linked that field successfully.
In database, useremail is NULL, and it’s NULL also by frappe.get_doc