No, your client’s zoho credentials are not to be used in order to log into the customer portal.
When you send a portal invite to the email address of your client and they accept it, they will be requested to create a new password using which they can log into the portal you’ve invited them to. Their Zoho credentials has no relevance to this, and even clients without a Zoho account can log into your portal using their email ID and the password they created.
Online accounting software
for small businesses.