Hey @Amir_Sharif! That field should allow you to specify the db you want to query (and override the default), but does not allow you to provide the SCHEMA. In your use case, you can leave fivetran_database in the Override DB name field, and then omit it in your query. But you would still need to reference the SCHEMA in the query ie: HEROKU_POSTGRES_PUBLIC.
ie: With fivetran_database as the new db name, your SQL should reference HEROKU_POSTGRES_PUBLIC.users