Answer
PO Processing Types only support automating the process up to printing the PO — they don't extend to automatically creating the receipt. The practical alternative is adding a custom Command Button (in the PO page's Status Command Group, for example) that navigates to the Register Arrivals screen pre-filtered by that specific PO Number — this doesn't fully automate the receipt itself, but removes the manual navigation and searching step, getting the user straight to the right pre-filtered screen from the PO page.