It's not too important when it comes with the watch-only wallet's functions since it can derive public keys and create unsigned transactions without it.

It's mainly a requirement of most wallets to verify that you're using the correct signer.
It's computed (hashed) from the master private key itself.

Here's how to actually compute it: /index.php?topic=5444628.msg61903879#msg61903879
It has to be the "m" and not any other child xprv derived from it.
And you need another tool to separate the "private key" from the "chain code" of your master private key (xprv),
The tool should base58 decode the xprv, remove the last 4-Byte checksum, the private key should be the 32 Bytes prior to those checksum bytes.