A customer key is a user-defined identifier for an asset. Each customer key is a unique value across your enterprise. The customer key isn’t a required field, and if you don’t provide a value, Marketing Cloud Engagement generates a unique GUID value for the customer key.
Add a customer key to the asset as a string with a 36-character limit, using the customerKey attribute.
Sample Asset with Customer Key
{
"name": "NTO Welcome Series Email",
"assetType": {
"id": 207,
"name": "templatebasedemail"
},
"customerKey": "090c69e7-59ac-4286-8733-f8f76example"
}