Custom claim not returning with OpenIDConnect url

I am authenticating using the Okta account and want the customized token containing the custom claim (upn) with it.
My application authentication url : https://dev-396276.okta.com/.well-known/openid-configuration?client_id=0oaeu4ckbqSc0GXxf357
this does not return “upn” claim.
But when I try to token preview for default authorization server, it does returns me with “upn” claim.

Need your guidance ASAP as we are planning to go for our customers live updates for SSO using Okta.