How do I get the purchaseToken after the IAP is completed?

Good morning/evening,

Using the Mobile IAP plugin, how can I get my hands on the purchaseToken after a successful purchase? The transactionID expression is not useful for my application. I understand there is the option of using a validator URL, but I would much rather just get the purchaseToken in an expression instead?

Also, if a validator URL is set in the properties, 'on transaction complete' never triggers, and I can never access the transactionID expression.