You need to add the property Conditinal Building = True and add "Reward" to the Occupant Groups. You also need to add a LUA script which will be examining the conditions and enabling/disabling the lot accordingly. And of course some LTEXTs for the user (eg displaying the reward notification or explain why the building is not available).
You can see these in my Chamber of Commerce and Industry (dl from the LEX or the STEX).
