If you integrate your product with your Receptive account, following the steps here, then your customer data will be automatically added to Receptive.
Once you expose a link to customers, they can then log in to Receptive through the widget or a link to the portal, depending on which option you chose.
If you don't want to integrate with Receptive, or want to get started in the meantime, it's possible to manually invite customers into Receptive.
Note: This isn't recommended for a long-term solution because your customer data won't be automatically updated and customers have to manually login to Receptive.
You can do this by following these steps.
You can set up SAML SSO to enable your customers to log in to Receptive without a username and password.
Follow the steps here to get set up.