In Thunderbird find the ≡ menu in the top right, choose "Options" then "Account Settings". You should get a window similar to the one below (although possibly pre-filled if you have other accounts.)
Under "Account Actions" choose "Add Mail Account"
Use your departmental username (As you'd use to log into a departmental computer) e.g. firstname.lastname@example.org
As Thunderbird doesn't know the default settings you have to change the hostname, SSL option and Authentication option so it matches the image shown above. Obviously checking the username matches your departmental username.