What is x-request-id in Apple Pay Merchant Token Management API

I'm currently implementing recurring donations for our Apple Pay on the Web integration and I'm trying to figure out what the x-request-id header is on the Get Details of a Merchant Token Event HTTP request. I'm not sure if that's an existing value I'm throwing in there or if it's a generic id I'm just putting in the header for Apple to use.

the documentation in question is:

Get Details of Merchant Token Event

Replies

I've run into exactly the same thing. The docs say it must be included in every request, but doesn't say what it is. I found a thread that implied that it could be a timestamp, but the description doesn't say what format it is expected to be in.

Did you ever find out what it's meant to be?