Please use the USB Type-C to 3.5mm adapter that came in the box with HTC U11 since it has a unique hardware design for HTC U11.
Or you can purchase the HTC USB-C Digital to 3.5mm Audio Jack Adapter on the HTC website.
Also make sure to update HTC's Audio Jack Adapter Toolkit on your phone to the latest version from Google Play™. Then plug in the adapter with your 3.5mm headset to your phone to check for its latest firmware update. Follow the onscreen instructions to update the firmware.