How to secure simple dropwizard Java REST API


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

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

If I use the same call as for ocidebugger, that is

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”

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?