This keeps happening to me as well, no matter what I type into the fields.
This is especially irritating as I am having to reinstall several times, testing various Blender 2.7/2.8 beta versions, on both Windows and macOS.
EDIT: WHOOPS! I figured out what went wrong. After reinstalling for the seventh time, I started Googling for "ProRender registration", then typing information into the resulting webpage.
However, if I click the "Register" button in the actual installer, the resulting page produces the needed code.