In short, I have the exact same problem as described in #1179 by @JamesTaylor04, unfortunately the author closed the issue and did not report what his solution was.
I configured OpenAPI as follows:

@OpenAPIDefinition(info = @Info(title = "application",
        description = "TODO", version = "v1"))
@SecurityScheme(name = "oauth2, type = SecuritySchemeType.OAUTH2,
        flows = @OAuthFlows(clientCredentials = @OAuthFlow(tokenUrl = "${spring.security.oauth2.client.provider.oauth2-provider.token-uri}")))
public class OpenApiConfig {
}

Swagger-Ui shows the authorize button, the form with the correct values for the token endpoint, client-id + client-secret.
But when I press authorize and it requests the POST https://{OAUTH2_SERVER}/realms/${OAUTH2_REALM}/protocol/openid-connect/token' endpoint, but then returns a CORS error:

Access to fetch at '.../token' from origin 'http://localhost:8080' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

The OAuth2 server is not hosted on the same domain as the Swagger-UI/Spring-Boot-Application.