Where do I find my subscription ID?
Sign in to your account and go to Account Settings. You'll see your subscription ID in the top right.

How do I know the credits were added to my account?
Go to your Account Settings and click on Plan Status. You'll see the number of credits you have in the top right, under Credits Remaining.

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You'll receive an invitation email from Esri Customer Service. Your subscription ID is in that email. You will need to activate your account first and then come back to this page to get your credits.