It seems that PM has changed the way you Refer-A-Friend. I was not able to locate any documentation on the new method so I've decided to share how it's done today.
In the past, you were able to use your friend's phone number that is with PM to use as the "code" in the Refer A Friend field (the field was made for a phone number as shown in this post here)
Today, it is a custom code found in your profile.
Login to your account, navigate to Overview tab. On the right-side of the page, you will see your custom code that is alphanumeric. Share this code or the custom link with your friend.

Old field:

New field:
