Establish a Confidential Client

Questions API
1 
    @PostMapping(path = "/verify", produces = {
                    MediaType.APPLICATION_JSON_VALUE })
    public ResponseEntity<Registration> verify(@RequestParam(name = "token") String activationToken,
                    @RequestBody(required = false) Registration registration, final HttpServletRequest request,
                    final HttpSession session) {

            final RequestContext requestContext = OktaUtils.constructRequestContext();
            requestContext.setUserAgent(request.getHeader("USER-AGENT"));
            requestContext.setIpAddress(request.getRemoteAddr());

            AuthenticationResponse beginResponse = idxAuthenticationWrapper.beginUserActivation(activationToken,
                            requestContext);

            log.info("Begin Response Status {}",
                            beginResponse.getAuthenticationStatus());

             return new ResponseEntity<>(HttpStatus.OK);

}

Okta returns error: Error Detail: [invalid_request:Only confidential clients can use the activation token flow]

Question: How do you establish a confidential client?

I have configured in the Admin Console with multiple different Application types with Client Secret and Interaction Code enabled but unable to determine how to establish a confidential client for the SDK (GitHub - okta/okta-idx-java: okta-idx-java).