If the answer to your question is a prepositional phrase, the question must also contain a preposition.
There are two different ways to do that, depending on if the answer involves a person or a thing.
When the answer will include a preposition + person combination, the question word becomes: preposition + wen/wem ("wen" for accusative, "wem" for dative).
When the answer will include preposition + thing combination, the question word becomes: wo + (r) + preposition.
For prepositions that start with a vowel (a,e,i,o,u), we have to include an "r" after "wo."
There is also a second version you can use, but it's very informal and some people think it sounds a bit "dumb."