How can I use CreateSignedJWT with a string rather than keystore key

I'm trying to do the following:

secret_key = "sometihng";
web_token = CreateSignedJWT(payload_string, secret_key, hash_params );

And I receive error: signOptions type not supported, it has to be of type Key or KeyPair or Struct

Coldfusion's jwt documentation states the secret_key parameter should be a struct, which I understand. However, I'm unclear on how to use the secret key as a string without using the keystore, because if I use

key_options = {
        key = secret_key
    }

web_token = CreateSignedJWT(payload_string, key_options, hash_params );

then I receive error Either the keystore path is invalid or corrupt or the keystore password is wrong.

The key itself is stored in an encrypted database column, so I want to pass a string version of it. How can I do this without the keystore?