From the city centre take bus Nr. 91 towards "Rossmarkt" (you can catch it on the opposite side of the main station, there's a small bridge that leads across the rails) and get off at "Wernweg". Here you are! This is a bus stop with plenty of space for cars to stop, right in front of the entry to the highway (A70, which leads towards A7 and A71). Plus there's a gas station where you can ask for a sign or ask drivers directly. There is a lot of local traffic, but I have also found it very easy to hitch from here towards Würzburg. Other directions could be slightly more difficult, but also possible. You can also catch a ride to "Autohof Werneck", which is only a few kilometers away, and use it as a relais. It is situated between different highways, and there will be drivers from the south going north (onto A71 towards Erfurt) and vice versa, as well as traffic to/from the east (A70 towards Bamberg). But since it is a "Autohof" it is not directly next to the highway, has less traffic, so prepare for longer waiting times.
In case you want to hitch further south, e.g. in the direction of Stuttgart, ask drivers going towards Würzburg to drop you off at the Raststätte "Riedener Wald". From there it is easy hitching long distance rides to the south. (There's a gas station and a restaurant).