"Send yourself a copy" field in contact form

Hello, does anybody know how to set a "send yourself a copy" checkbox field in drupal contact form. This field is visible when administrator is logged in, but I would like to set this field for a global contact form on a site. I would like everyone who will be sending a message to admin using a contact form, would chose, whether he or she wants to send themselves a copy of their message. And I'm talking here about anonymous users of course. Any suggestions? Maybe there is a module? I tried a Webform, but didn't find the solution :(