How to forge RS256 JWTs

I need to forge invalid but verified JWTs to test our JwtValidators in Spring Boot services.
I’ve learned how to forge HS256 JWTs by using the secret found at:

SELECT value
  FROM component_config CC
  JOIN component C ON CC.component_id =
 WHERE C.realm_id = 'MyRealm'
   AND C.provider_id = 'hmac-generated'
   AND = 'secret'

How can I do the same with RS256 JWTs? What can I do with the value found with the query:

   AND C.provider_id = 'rsa-generated'
   AND = 'privateKey'

Where do I find the value to put here (

Nobody has ever forged JWTs?