There is no convenient hitchhiking spot on the motorway in Witten itself. Witten borders Dortmund and it is only a few minutes by train to go there. From there you have good hitching possibilities in all directions. Check the article of Dortmund for detailed information.
Have a look at the article of the Ruhr Area to see why it can be very difficult to find a lift exactly to the location you want to go inside this area. You should accept rides to cities next to Witten as Dortmund, Hagen or Bochum and finally go to Witten by public transport.