When purchasing vSAN Addon licenses, they are added to your licenses as a separate license, and a cluster can only have one license applied to it.
When purchasing a new license, it will be a separate license that you can find in your portal.
If needing to apply an addon license to increase licensed capacity for your vSAN cluster, you will need to merge it with your vSAN license currently in use. This can be done within your customer portal in the license section. Please see the following: License Key Management for VMware Products
For steps on how to merge the licenses, please follow this documentation: Merge and split license keys for VMware products