Steps to reproduce on regtest:

```
converttopsbt 02000000000101ab73dba46508bacdb49a988ddd74bc9d39c9cb7e057af060ca8c6b6861e28e3c0300000000feffffff02b828d0b20000000017a914d1954862afd728fbb055149002958e1d1b9e1f0b8700ca9a3b0000000017a91442170ccb3441cff1723af9afab6f2553103e1d578702473044022077380c6e16496d9905aef120694c329be51522847cb5d53fa74b94698c95678d02202d32c46e0e17420a83dee830fefda7d40529182f5d82be47c2b49a7b83214ab50121026d483d6628dcc3ef6e3ab9ed4937c452c263c7e0ac80eb4be97426d5da8f640500000000
```
or 
```
converttopsbt 02000000000101ab73dba46508bacdb49a988ddd74bc9d39c9cb7e057af060ca8c6b6861e28e3c0300000000feffffff02b828d0b20000000017a914d1954862afd728fbb055149002958e1d1b9e1f0b8700ca9a3b0000000017a91442170ccb3441cff1723af9afab6f2553103e1d578702473044022077380c6e16496d9905aef120694c329be51522847cb5d53fa74b94698c95678d02202d32c46e0e17420a83dee830fefda7d40529182f5d82be47c2b49a7b83214ab50121026d483d6628dcc3ef6e3ab9ed4937c452c263c7e0ac80eb4be97426d5da8f640500000000 false
```

Further reading: #13738
}
    if (request.fHelp || request.params.size() > 2) {
        throw std::runtime_error(
            RPCHelpMan{"sethdseed",
                "\nSet or generate a new HD wallet seed. Non-HD wallets will not be upgraded to being a HD wallet. Wallets that are already\n"
                "HD will have a new HD seed set so that new keys added to the keypool will be derived from this new seed.\n"
                "\nNote that you will need to MAKE A NEW BACKUP of your wallet after setting the HD wallet seed." +
                    HelpRequiringPassphrase(pwallet) + "\n",
                {
                    {"newkeypool", RPCArg::Type::BOOL,  "true", "Whether to flush old unused addresses, including change addresses, from the keypool and regenerate it.\n"
            "                             If true, the next address from getnewaddress and change address from getrawchangeaddress will be from this new seed.\n"
            "                             If false, addresses (including change addresses if the wallet already had HD Chain Split enabled) from the existing\n"
            "                             keypool will be used until it has been depleted."},
                    {"seed", RPCArg::Type::STR,  "random seed", "The WIF private key to use as the new HD seed.\n"
            "                             The seed value can be retrieved using the dumpwallet command. It is the private key marked hdseed=1"},
                },
                RPCResults{},
                RPCExamples{
                    HelpExampleCli("sethdseed", "")
            + HelpExampleCli("sethdseed", "false")
            + HelpExampleCli("sethdseed", "true \"wifkey\"")
            + HelpExampleRpc("sethdseed", "true, \"wifkey\"")
                },
            }.ToString());
    }
    if (pwallet->chain().isInitialBlockDownload()) {
        throw JSONRPCError(RPC_CLIENT_IN_INITIAL_DOWNLOAD, "Cannot set a new HD seed while still in Initial Block Download");
    }
    if (pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Cannot set a HD seed to a wallet with private keys disabled");