have a question :check:construction=> Which is the field of the privatekey

hello my friend
in /construction/combine , where is words write in privateKey ? in config.json ：
`"prefunded_accounts": [
{
"privkey": "privkey",
"account_identifier": {
"address": "address"
},
"curve_type": "secp256k1",
"currency": {
"symbol": "ELF",
"decimals": 8
}
}
]
},`
but do /construction/combine signing,The privatekey of the signature is not passed in config.json, and there is no specific information in the official document.