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".