First of all, make sure your webhook is set up to receive notifications. You can receive the recurringDetailReference in the AUTHORISATION and the RECURRING_CONTRACT notifications. 

For the AUTHORISATION notification, you must:

1. From the Customer Area, open your company-level account.
2. Browse to Developers > Additional data > Payment and enable Recurring details.
3. Once this is done, in the AUTHORISATION notification, within the additionalData, you will receive the shopperReference and recurringDetailReference:

1{

2   "additionalData": {

3      "recurring.shopperReference": "testshopper",

4      "recurring.recurringDetailReference": "1111111111111111"

5   }

6}

For the RECURRING_CONTRACT notification, you will need to enable the non-default event code, as explained here.