Ask the hotel where you staying they may do it for you
Many buses head to lushoto but almost all leave early 6~8am
Hello Matthias, have you visited already? Yes it is possible to get a bus ticket before visiting the Bus Terminal. These folks in Dar can do just that for you at No Extra Costs https://www.tripadvisor.com/Attraction_Revie… Try calling or email them.
The Luxurious 2x2 Sit Bus such as Shambalai costs 9$ for a Ticket from Dar to Lushoto which is the same as 18,000TZS just note the bus actually leaves 6:00AM