If you are using a 4412D+ or 4424D+ you can press Menu Twice, then the More arrow, then HDesk is an option. Choose Logon and it will ask for the extension and password. You can logoff the same way.
The other option is to program buttons on the phones. Extn Logon and Extn Logoff are options under the advanced button features. Remember, you need the Logon button on the phone that is being used to log-onto and the logoff button on the phone that is logged in. If possible, I would put those buttons on all telephones associated with logging in/out to avoid confusion.
If ext. 200 is the desk you want to log ext. 3400 onto, ext. 200 must have the logon button programmed and ext. 3400 must have the logoff button programmed.