You enter the XPRV (which is short for "Extended private key"), encoded in base58 format. It's the same encoding used for XPUB ("Extended public key").
I suggest spending some time on https://iancoleman.io/bip39/ to get a better understanding of keys and encodings. Try generating a few keys. WARNING: Only use the site for learning; don't use it to generate keys for a real wallet!
The XPRV for the master key is labeled "BIP32 root key". XPRV strings have the prefix "xprv".
You enter the XPRV (which is short for "Extended private key"), encoded in base58 format. It's the same encoding used for XPUB ("Extended public key").
I suggest spending some time on https://iancoleman.io/bip39/ to get a better understanding of keys and encodings. Try generating a few keys. WARNING: Only use the site for learning; don't use it to generate keys for a real wallet!
The XPRV for the master key is labeled "BIP32 root key". XPRV strings have the prefix "xprv".