Here is the sample API for your reference. You have to define confiTemplate parameter with pre-defined template name eg: "DATABASE-TEMPLATE-2".
"type": "OracleProvisionParameters",
"container": {
"type": "OracleDatabaseContainer",
"name": "vdbname",
"group": "GROUP-20"
},
"source": {
"type": "OracleVirtualSource",
"archivelogMode": true,
"configTemplate": "DATABASE_TEMPLATE-2",
"mountBase": "/mnt/delphix"
},