I don't want to get only (r,s) pair, because it is trivial. I want to get data, behind a given z-value.
-snip-
And it is not just a transaction hash. Instead, it is that thing:
Yeah, I'm aware that it's not the transaction's hash; in fact, I demonstrated how to do it here:
https://bitcointalk.org/index.php?topic=5479862.msg63427494#msg63427494I thought you only need the signature which is what you've asked in the OP.
Anyways, AFAIK, 2coins.org's online rsz tool that can do exactly that but it's exclusive to legacy transactions.
Paste an existing TXID and it'll automatically fetch the transaction from various blockchain API then compute the message hash.
But I'm not aware of any that will work on SegWit and TapRoot transactions.