First part is to create the promotion code.
1. Navigate to Lists > Marketing > Promotion Codes > New.
2. In the Promotion Code field, enter a name, number or code for this promotion.
3. In the Discount field, select the discount item you want to assign to this promotion code.
4. In the Apply Discount To field select the discount application on Sale (First Sale Only, All Sales).
5. Enter the Start Date and End Date for this promotion.
6. Uncheck Available to All Customers.
7. Click Save.
Second part is to create the Group.
Third part is to create the Marketing Template
Fourth part is to create the Marketing Campaign.
1. Navigate to Lists > Marketing > Marketing Campaigns > New.
2. Click the Primary Information tab.
3. Enter an ID for this campaign.
4. Enter an Title for this campaign.
5. Under the Events tab, click the Email subtab.
6. In the Target Group field, select a group.
7. In the Email Template field, select the template.
8. In the Promotion Code field, specify the Promotion Code.
9. Click Save.
When a user creates an Invoice or Sales Order in NetSuite after the campaign is sent, the user can select the promotion code in the promotion code field. This promotion code is available only to customers who are member of the group used in the campaign.
Note: When creating a Promotion Code, user can choose not to check the Available to All Customers checkbox and not associate any Partner (the online help gives the impression that you must do one or the other. Help Document Path: Marketing, Sales Force Automation, and Partners : Marketing : Setting Up Marketing : Promotion Codes)
Instructions on how to create group and marketing template is available in Help Guide : Working with Groups and Email Marketing Templates.