To enable Self Service for the Person from CRM, you will need to navigate to the Self Service tab under the Person context. Then you can check the Self Service Enabled checkbox which is available on the Self Service Details screen and click on save button in order to activate the Self Service for that person as shown in the below screenshot.
After activating the Self Service as explained above the person for whom self service is enabled can login to support site using the above given credentials.
Now suppose, you want these credentials to be customized the way you want to take the default values. What you can do to incorporate this is, you can write a script on the Custom Content area of the above screen, but you might not have observed that this screen is not customizable at all unless the self service enabled checkbox is checked and values are saved at least once. Whatever script we have written on the above screen will not work if the checkbox is not checked on the on load event itself and as we are not having control over screen, we cannot use on load event. Instead, what we can do is we can give a provision for enabling Self Service for the person from Person Entry screen itself as shown in the below screenshot.
In order to achieve the same, we can do some customization in Person area. We can add a check box called Self Service Enabled on the Person Entry Screen. If user will check this checkbox, then we will enable the Self Service for that particular person, so next time when self service tab loads the checkbox will be checked by default and the screen will be customizable according to your requirement.