I also had the same problem as a external service required a HMac signed token. It's already time ago whe this quetion has been raised, but for the sec of somebody needs a complete solution.

This is a complete code which you can copy into a function rule and is should work immediately.

You need to include the following packages in your library which you will use for the fuction rule:

java.util.*

com.nimbusds.jwt.*

com.nimbusds.jose.*

com.nimbusds.jose.crypto.*

/* Code Block START*/

JWSObject signedJWT = null;
try{

  // Create the JSON JWT Header
  JWSHeader header = new JWSHeader(JWSAlgorithm.HS256);
  
  // add expiration time (in milliseconds)
  long expL =  new Date().getTime();
  expL = expL+90000;

  // Create the JSON JWT Payload
  JWTClaimsSet claimsSet = new JWTClaimsSet.Builder()
    .subject("/your Subject")
    .expirationTime(new Date(expL))
    .issueTime(new Date())
    .issuer("yourIssuer")
    .jwtID(key)
    .claim("uid", "some claims you want") // append as much as you want before calling build(), like .claim("","").claim("","") as the return is always the builder object.
    .build();

  signedJWT = new SignedJWT( header, claimsSet );

  // Sign with the HS256 key passed as the string parameter
  signedJWT.sign( new MACSigner(key.getBytes()) );
  
}
catch ( Exception e )
{
  oLog.infoForced("CustomUtils generate Token with HMac: " + e.getMessage() );
  return "";
}

return signedJWT.serialize();

/* Code Block END*/