I need to create a password field for customers
If you want to create a custom field that will store the password then there isn't a specific custom field. You can use a text or number custom field which will display the contents entered, unlike a password field. The custom field is used to capture and retain additional information about the subscriptions or customers. To create a custom field, head to Settings > Configure Chargebee > Custom Fields. Read about custom fields here.
Enable the configuration setting for password. Settings >> Configure Chargebee >> Checkout & Self-Serve Portal >> Allow customers to log in to the checkout and portal using >> select Password or OTP.
To set a password for your self-serve portal, your customers need to log in to their portal using an OTP.
Next, select Account information > Set up Password.
Your customers can log in to their portal using this password next time. Please refer here for more details.