Dax is a city in France
All buses cost 1€. You can use Google Maps or Moovit to plan your bus ride but sometimes the timings are messed up. Use these pdf schedules which will be more accurate.
Take the bus nº2 heading to Bruyères and get off at Pierre Benoît and walk straight along the street for around 400m until the second roundabout. There's a sidewalk, a light so you can hitchhike at night if there's traffic and space for people to stop. You'll find traffic going to Bordeaux here, quickly if it's peak traffic hours but don't go to Castets (which is where the A63 entrance is) because cars can't stop there and you'll wait for a long time.
Take the bus nº1 heading to Barradeau and get off at La Pince Camot. Walk 1,2Km until this roundabout: 43.725705, -1.098425 . This is a departmental road that goes to Bayonne, it has a lot of roundabouts and most people go to towns. You can find a lot of people that go on the A63, but if I can't find traffic going there what I usually do is go to Benesse-Maremne and then hitchhike to the toll, which is very easy and quick.