First go to the Contacts section and search for your customer.
Then click on “Edit Company” button on toolbar. Click on Customer Specific Pricing tab.
Click on Add Manufacturer button and find the manufacturer you want to add.
Insert a new Discount and or Markup for your customer. When complete click on OK button to save. This will override whatever standard discount and or markup you may have as a default.
Please review the attached snapshot for more details.