Function import_key 

pub(crate) fn import_key<D: Equivalence>(
    cx: &mut JSContext,
    global: &D::GlobalScope,
    normalized_algorithm: &HmacImportParams,
    format: KeyFormat,
    key_data: &[u8],
    extractable: bool,
    usages: Vec<KeyUsage>,
) -> Result<DomRoot<CryptoKey<D>>, Error>

https://w3c.github.io/webcrypto/#hmac-operations-import-key