To get past the "Trust this Computer" screen you must navigate to the iPad in the locations section of finder on your Mac while the iPad is connected:

Once you hit that trust button on the Mac the prompt should appear on the iPad and you will be able to share.

Note if you are using your iPad to screenshare your dock will likely not provide enough power to keep the iPad charged. I would suggest either using the wireless connection or running the iPad direct in the usb C on the Mac (you will need a usb>lighting cable or adapter to do so).