You need to land at a Port of Entry for US Customs clearance. It is also necessary to identify the nearest office from your departure location in your eAPIS manifest (the "Departure Airport").
The information is sorted by state, which then brings up a list of offices. A given office may cover a few airports, so it may not be obvious at all that Oakland County (Pontiac) is available, since it is listed under the Detroit Metropolitan Airport office. Similarly, Watertown, NY is listed under the Alexandria Bay office. So choose an office vaguely near where you want to go, at look at the list of all the Facilities and Crossings they handle.
Note that the search box searches the entire CBP website, not just the ports of entry. With a search for "Oakland County", the first result is Detroit Metropolitan Airport and clicking that gives you the office for that Port of Entry, so that is another way to find the office that handles the airport you desire.
Since the web site may be out of date, just call and ask questions. The officers are very helpful !