This article explains how to find the subscription ID for your Microsoft 365 subscriptions.
Log into your Microsoft 365 account. Go to the Admin Center.
Go to Billing > Your Products. Select the desired product.
Locate and copy the subscription ID from the page url.
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See Find the Subscription ID of your Microsoft products (Dynamics 365 Customer Engagement Plan) from Nishant Rana's Weblog for more information.