Face recognition is turned on automatically in Elements 2019. However, depending on how many photos you have in your catalog, it can take quite a while to complete the analysis. Once the photo has been analyzed, you can hover your mouse over the face in the scanned photo, and if you are lucky, the person's face will have a suggestion for who the person is based on photos you have previously identified as the person. If the face is not recognized from previous photos, you will see the Add Name dialog and you have to add the name manually.
The more efficient way to complete face recognition is to look at the stacks under the Unnamed People tab.
I'm not sure if I am answering your question. If you don't recognize the person in the scanned photo and are trying to have Elements figure out who he/she is, it may not be able to do what you want. Although face recognition gets better each year, it is by no means perfect. I have more than 10,000 images of my daughter where I have named her face. Yet, new images fail to recognize her as my daughter.