How to secure simple dropwizard Java REST API

Hello,

I am new to okta OAuth2.

I try to secure my single dropwizard REST Api service using okta OAuth2.

I manage to set up the service side with the use of a how to guide, test it successfully with a token which I generated in setting up a web app in okta and then with the help of the site https://oidcdebugger.com/debug.

However, I am completely at a loss on obtaining a token from postman.

If I use the same call as for ocidebugger, that is
oauth2/default/v1/authorize?client_id=XXX&redirect_uri=https://localhost:8443&scope=openid&response_type=token&response_mode=form_post&state=ZZZ&nonce=something

It keeps me asking to login.

But providing username and password in the header wont do. (The web app approach seems to rely on a session cooky)

I tried various other calls, eg: “Get Access Token with Resource Owner Password Credentials”
{{url}}/oauth2/v1/token

Here I manage to obtain a token, after fiddling around with creating a native app, an configuring it to accept username/password in okta.

But this token will not authenticate my service:

Signed JWT rejected: Another algorithm expected, or no matching key(s) found

Can anyone point me to a concise how to?

Thanks

Matthias