HTC U12+ doesn't have a USB Type-C to 3.5mm adapter in the box

Available for pre-order in the US as of earlier this week, the HTC U12+ is a powerful, high-end flagship that replaces last year's U11. Similar to the U11, the HTC U12+ does not feature a standard 3.5mm headset jack, but it comes with HTC USonic Adaptive Earphones in the box - these connect to the handset via its USB Type-C port.
The retail box of the HTC U12+ that's sold stateside contains an HTC Rapid Charger 3.0, a USB Type-C cable, an HTC Clear Case, and the aforementioned USonic Adaptive Earphones. So, if you want to use 3.5mm headphones with the U12+, you'll just have to buy an adapter yourself (HTC is selling such adapters for $12 a piece).
source: HTC (Twitter)