You walk (a very nice walk) until the gas station Eni Tankstelle (Oiden 110, 3300 Amstetten, Austria). Once there you skip this awful station and continue to the roundabout. I promise you the spot is much better than this gas station, so skip it. In the roundabout you turn right, to the highway direction Linz / Wien. After 20m you will find a very nice spot that looks like a bus stop. I have never waited there more than 20 minutes. Also I would highly advise to use a cardboard because people can drive anywhere.
If you are going to Vienna, avoid middle stops like Sankt Pölten. Its an awful place to hitchhike because it's out of the highway. I have always end up finding someone who goes straight to Vienna.
Places to avoid
Hitchhike in the Eni Tanktelle gas station.