You may have been asked to find your User ID (AKA your Referral ID) for customer support issues. Finding it is easy!
On the web:
1. Log in and hover over your email address in the top right corner of the screen to see a drop-down menu. From there, you can click Settings. Then go to your Basic Info page and you'll see your User ID near the top of the page.
On the mobile app:
Login and tap the profile icon 
in the bottom right corner. Your Referral & User ID can be found at the top of this section.
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