I solved this by using the interval popup. I selected the F and then added a note a major 3rd below. When I used the mouse, it was attaching it to the the bass staff.
I tried the Alt-N, but it would move the note to the top of the stack.
I also have started adding notes with the keyboard, as you suggested, Daniel.
This is my first song with Dorico. I wrote it at the piano, and arranged it on manuscript paper. Still have a lot of tutorials to watch!
Thanks again, dankreider and Daniel.