How to set Bitcoin-Qt setgenerate mining recipient?
By default, Bitcoin-QT will generate new addresses to mine to.  If you use the Encrypt Wallet feature, it appears to always mine to the same address instead, but it's still not user-selectable.

If you do want to CPU mine to a specific address (e.g. for testing on the RegTest mode), you can use mining software like cpuminer/minerd in "solo mining" mode to do so.