The Contacts Sidebar is shown below.
Opening
–Menu Bar > View > Contacts Sidebar.
–Composition Toolbar > Contacts.
–Hotkey: F9.
Elements
–Button ×: Close the panel.
–Address Book menu: select an address book to search for contacts.
–Context menu ☰: set the selected directory as the default directory when opened.
–Search Contacts field: enter the name or address of the contact to search for.
–Add to To, Add to Cc and Add to Bcc buttons: add the selected contact to the corresponding field of the e-mail.