Log into Microsoft Volume Licensing Service Center portal – VLSC, and open the Relationship Summary page.
Select ‘Add Open License’ and enter the License & Authorization numbers that you would have received from your Microsoft licensing reseller. You’ll need to wait for the licensing request to be processed before proceeding.
After processing you should receive a product key for the purchased Azure credits. Go to the following url to add the credits – https://account.azure.com/Subscriptions
Click on ‘Click here to add credits’ and enter the product key.
If the product key is accepted confirmation will be displayed.