One of the options to create and also sign in to Lifesum is using the option to log in with your Apple ID in two different ways:
 
  1. Showing your actual email address connected to your Apple ID under Settings -> Account Settings in Lifesum.

  2. Using Apple’s "private email relay" service and hiding your email address under Settings -> Account Settings in Lifesum (the field will show an email address created by Apple ending with "@privaterelay.appleid.com" (emails will then be routed to the Apple ID’s verified email address).
 
The email address you are using for Lifesum can be found under Settings -> Apple ID -> Password & Security -> Apps Using Your Apple ID -> Lifesum.
 
Please find more information here: