Walk northwest on Avenue Mohammed V, which becomes the N10. Once you reach the edge of the city you will come to a Shell station and will see the airport to your left. This is the best spot to hitch from.
Also you can take bus №1, when you'll see the sign - "Errachidia", bus turn left, this is your stop. Walk Errachidia road about 500 metres and you'll see petrol station and a good stop to hitch.