To modify the "sub" field in the id token in the trial version

I want to change the value of the “sub” field in the id_token. Can anyone suggest me the expression for the providing the idpuser_id as the value of the “sub” field?

Modifying the sub claim is not supported for ID Tokens. The sub is a reserved claim and, per the OpenID Connect spec, MUST be locally unique.

Okta will always set the User’s ID (in Okta) as the value for the sub claim in the ID Token, as mentioned in our documentation here.

You can modify the sub claim included in the Access Token, but only if you are using a Custom Authorization Server. You should see it as the only/first pre-configured claim in a custom server.

Actually I wanted spaSigningCertificate just to verify my jwt token but in dev-okta dashboard Im struggling to find the certificate that I’ll pas in the method call of jwt verification. That’s why I wanted to change the token with my own id as sub field. As per you say it won’t work now. I’ll try another way.
Thanks for responding.