A very common and useful construction is to create a link a user can click on to open a new window. Not only is a new window opened, but values from the current form are passed so that the window can perform a query based on the passed values.
The first step is to use the HTML A tag. This is placed on the originating page. You have considerable control over how this link is displayed, from color to the style of the cursor.
The values of cus_number and order_number will be passed to the Discounts window. Note however, these values must exist or be passed from the originating page or the onClick event will fail.