Keep in mind that first, you need to create your formula for a discounted price on the Formulas section, set under Products in your AppScenic Account. 

Check here how to create a new Formula.

Now that you have your Formula done, go on your AppScenic Account, click on Products and then on My Products.

Here, find the products you want to add to your formula and check the box near each of them.

Choose Update Price Formula from the actions on top of your products list. 

Then check Add Price Formula, choose the formula you want and Save.

And now you know how to apply a discount price to multiple products using the AppScenic Formula tool!