How to send customerId (x_cust_id) to authorize.net ARB?

I know, this is [normally] an RTFM question, except, well, we're talking about Authorize开发者_如何转开发.net so the FM is hardly R-able.

I noticed when looking at the all the fields that get sent during the Silent Post, Authorize.net returns an item called "x_cust_id". It is not addressed in their sample code, and I can't find out where in the XML document that field would get posted. I'd like to use it!

So, how exactly do I add "customer Id" when I create a new ARB subscription?

Furthermore, is there any documentation that actually shows all the information I can send to ARB? I don't even know what my options are!

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Silent Post != ARB. They are two different things. Silent Post is triggered whenever a transaction is processed by Authorize.Net. They send the results of each transaction to a script on your server notifying it of the results of the transaction. This allows you to manage subscriptions that fail programatically. It is very similar to Paypal's IPN. x_cust_id will be populated from the <customer/><id/> field in the ARBCreateSubscriptionRequest API call.

FYI, this is explained on pages 20 and 21 of the ARB guide.