Step-1 : General tab
In this case, you will have to choose "Dependent product based discount" as discount method.
Step-2: Condition tab
In this example, we are going to offer 50% discount for headphone if you buy Laptop. So choose,
Product(s) from - Laptop
get discount in - Headphone
Step-3: Discount tab
Enter the discount value 50 in percentage and save the rule.
Step-4: Frontend demo