You can start hitchhiking just outside the city center e.g. on the road Leof Soudas (35°29'26.6"N 24°03'02.1"E) in the direction Souda. There are many possibilities that somebody would take until one of the enter of the highway.
You can walk or take a bus until the entrance of the highway.
It might work to hitchhike on the highway as there are also some bus stops along the highway.