from the existing
                 keypool will be used until it has been depleted.
2. seed          (string, optional, default=random seed) The WIF private key to use as the new HD seed.
                 The seed value can be retrieved using the dumpwallet command. It is the private key marked hdseed=1

Result:
null    (json null)

Examples:
> bitcoin-cli sethdseed 
> bitcoin-cli sethdseed false
> bitcoin-cli sethdseed true "wifkey"
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id": "curltest