Create a Global Discount Item Where the Price Level of the Latter Item is Set as Custom by Default when Added to Transactions

An alternate solution is to customize the entry form used to create Discount Items by hiding the Rate field in the form.

Steps  to Customize the form:

-Navigate to Setup > Customization > Entry Forms

-Locate the form where:

a. Type = Item

b. SubType = Discount/Markup

-Click Customize link or Edit if there is already an existing Custom Form used and navigate to Fields tab > Main subtab: unmark the box under Show column in line with the Rate field.

-Hit Save button

Once the form is customized, use the latter form to create the global Discount Item