I gather this window box is going to hang outside one of your windows? If that is the case, then you will want to use the strongest brackets you can find, preferably metal ones that will be solidly attached to the outside of the building. The last thing you would want would be for your box to fall onto some person or vehicle in the street below. Window boxes can be quite heavy when the soil is wet. Here is an example of a window box attached to a brick building:
I doubt that even the strongest window box brackets will support the weight of a person, although I certainly could be wrong about that. Even so, a box attached in a way such as is shown in the picture above should not interfere with the closing and locking of your windows.