https://travis-ci.org/github/bitcoin/bitcoin/jobs/680452476#L6107
HelpExampleCli("signrawtransactionwithkey", "\"myhex\" \"[\\\"key1\\\",\\\"key2\\\"]\"")
            + HelpExampleRpc("signrawtransactionwithkey", "\"myhex\", \"[\\\"key1\\\",\\\"key2\\\"]\"")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    RPCTypeCheck(request.params, {UniValue::VSTR, UniValue::VARR, UniValue::VARR, UniValue::VSTR}, true);
    CMutableTransaction mtx;
    if (!DecodeHexTx(mtx, request.params[0].get_str(), true)) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
    }
    FillableSigningProvider keystore;
    const UniValue& keys = request.params[1].get_array();
    for (unsigned int idx = 0; idx < keys.size(); ++idx) {
        UniValue k = keys[idx];
        CKey key = DecodeSecret(k.get_str());
        if (!key.IsValid()) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid private key");
        }
        keystore.AddKey(key);
    }
    
    std::map<COutPoint, Coin> coins;
    for (const CTxIn& txin : mtx.vin) {
        coins[txin.prevout]; 
    }
    NodeContext& node = EnsureNodeContext(request.context);
    FindCoins(node, coins);
    
    ParsePrevouts(request.params[2], &keystore, coins);
    UniValue result(UniValue::VOBJ);