Want to deliver emails from my onprem exchange and then forward email to my zoho email accounts.

I have read thrue the instruction  https://www.zoho.com/mail/help/adminconsole/configure-dual-delivery.html

But i cant see how to configure the contact to know it should send the email to zoho? Since all my users in zoho has only one email address and its the same as onprem exchange.

Regards

Tatsumi