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 Store. 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.