I’m bringing this back up because I have a similar question and I think I know the answer: you can’t. It’s one of the reasons I refuse to go to a non-3D Touch device because you can’t select text using the keyboard virtual cursor. Yes, you can hold the spacebar and move it but you can’t select text using the virtual trackpad from the keyboard without 3D Touch.
This also makes typing and selecting text in web forms like this more tedious and not as fun and precise as using 3D Touch since the cursor now sucks with iOS 13.