Create the Domain ID.
In the Inventory Section, open the:
POST /v1/inventory/userdomains/{domainName} Create a user defined domain group

Enter a name in the "domainName" field and click the "Try it out!" button.
In the Response Body, there will be an "id":

Use this id number in the "DOMAIN_ID" field for the creation of the plugin. For example:
{
"PLUGIN_CONFIG": {
"APIC_HOST_IP": "192.168.254.212",
"APIC_HOST_aaaUser_NAME": "api",
"APIC_HOST_aaaUser_PASSWORD": "apiReadOn1y",
"APIC_MAX_PAGE_SIZE": 15,
"PROTOCOL": "https",
"INVENTORY_POLL_RATE": "0 */10 *",
"INVENTORY_DELTA_TIME": 600,
"PERFORMANCE_POLL_RATE": "0 */5 *",
"PERFORMANCE_DELTA_TIME": 300,
"AVAILABILITY_POLL_RATE": "0 */5 *",
"AVAILABILITY_DELTA_TIME": 300,
"FAULT_SUBSCRIPTION_ENABLED": true,
"EVENT_SUBSCRIPTION_ENABLED": false,
"HISTORICAL_EVENT_DAYS": 1,
"DOMAIN_ID": 618 }
}