How to get "Prove Transaction hash" for a L2 withdrawal?

[gndelia]

Not an actual bug (I think, unless the feature is missing), but a question. According to this, given a L2 transaction hash for a withdrawal, we can use

const l1Rcpt = await crossChainMessenger.getMessageReceipt(l2TxHash)
const l1TxHash = l1Rcpt.transactionReceipt.transactionHash

and l1TxHash will be the finalized tx hash on L1. How can we get the Tx hash of the "Prove Withdraw" operation in L1 ?
If the withdrawal is not finalized, would that l1Rcpt.transactionReceipt.transactionHash be empty, or would it be the Prove hash? If so, how can I get both transaction hashes once the withdrawal is finalized?