services.bitcoind.<name>.rpc.users.<name>.passwordHMAC

Password HMAC-SHA-256 for JSON-RPC connections. Must be a string of the format <SALT-HEX>$<HMAC-HEX>.

Tool (Python script) for HMAC generation is available here: https://github.com/bitcoin/bitcoin/blob/master/share/rpcauth/rpcauth.py

Type
string matching the pattern [0-9a-f]+\$[0-9a-f]{64}
Example
"f7efda5c189b999524f151318c0c86$d5b51b3beffbc02b724e5d095828e0bc8b2456e9ac8757ae3211a5d9b16a22ae"
Declared
<nixpkgs/nixos/modules/services/networking/bitcoind.nix>