How do I connect an external gps antenna to my 67i Garmin ?

The 67i doesn’t have an external antenna connection. I think the 64 series was the last to offer that.

No it’s not possible to connect an external antenna to the 67i. The usb-c is only for power or data transfer, it has nothing to do with the antenna of the device. Why do you need an external antenna. Mine works fine without it. It’s meant to work with a clear view of the sky, outside, anything else is just a bonus. Mine works fine in my backpack and in my house, but like I said that’s a bonus, not a Garmin guarantee.

I need to operate the GPS (67i) in airplane and helicopter, but it cannot work without an external antenna 

Ok well Garmin makes aircraft portable gps units. Maybe they have an external antenna you can get for them, I’m not sure, but unfortunately the 67i doesn’t support an external antenna.

I checked Garmin aircraft products, they seem to have newer equipment now so I don’t see what I mentioned to you. You can still get the 64 series with an external antenna, but it’s not an inreach equipped gps unit. Check out the Montana 700i or the Tread series they  can be equipped with external antennas, and have inreach capability, but they are power hungry units so you’ll probably want to be able to keep them plugged into a constant power source all the time.