is it possible to use the apply to transaction's external id in the <apply> section of a Payment, Credit Memo, etc.? For example:

I've not tried that, but according to the API schema documentation, doc is an 'long' (not a record ref type), which i'm guessing only takes the InternalID of the 'applied' transaction. (https://system.netsuite.com/help/helpcenter/en_US/srbrowser/Browser2022_1/schema/other/vendorpaymentapply.html?mode=package)