On the instance's control panel, scroll down to the “Websites” section:

This section will display a table of all the vhosts that are declared on your instance, as well as the date that you added them. You can add and delete addresses from this interface.

To add an address, click on the “Add” button on the lower right of the table, which will cause the following prompt to appear:

Enter the website address that you want to associate with your instance (without the http://) in the “Address (vhost)” field.

When you click Submit, a message will be displayed that will tell you how you should proceed.

The following will depend on whether or not your domain is at Gandi or not (see more on this here)