How could I enable Sandbox for Care Identity Service (CIS2)?

Hi
I am developing a feature to login with NHS care Identity.
I tried to develop indirect request.
for example,
https://am.nhsdev.auth-ptl.cis2.spineservices.nhs.uk/openam/oauth2/realms/root/realms/oidc/authorize?response_type=code&scope=openid%20profile&client_id=&state=af0ifjsldkj&redirect_uri=<redirect_uri>

but I got the following error after go the link

invalid_client Client authentication failed

I believe it is because my sandbox app does not have access right to Care Identity Service (CIS2).

I already added Identity Service (Sandbox) in Connected APIs. However, it seems it does not enable Sandbox for Care Identity Service (CIS2).

ref: indirect request

Have you onboarded with the CIS2 Authentication service?