Introduction
When Impersonating an account, an admin is able to apply coupon codes and discounts for the user. There is exists a config which allows the admin to actually search for the coupons and apply their related codes directly, without necessarily knowing the codes beforehand.
Getting Started
Ensure the Configuration "Allow admins to search for discount codes when impersonating" is enabled.
To do this Login into your administrative console > Settings > Setup > Configuration > Search for the config and turn the toggle On. Select Save once you finish.

Steps:
First, you will need to navigate to the Account's page by logging onto your administrative console > Accounts > search for and select the Account you are looking for. Once you manage this, select the Impersonate Button in the far right column.

Once on the account's cart-side, If you haven't already, select Add to Cart on the item which the customer is looking to buy.

Select checkout to continue on with the process.

You will also need to continue through the purchase process until you reach the Payment Page.

Once at the payment page, scroll until you reach the Promotions and Coupon Codes section.

With the config turned on, you can simply search for the coupon using key words and select whatever coupons appear in the results.

By selecting the coupon from the search results, It automatically calls the coupon code associated to the coupon in the coupon code text box. From here you are able to actually apply the coupon code generated from the coupon search.

There will be a confirmation message confirming the coupon is applied. It will also inform you how much the coupon itself takes off from the customer's purchase.

Of course the coupon will now appear in the Cart alongside the final cost of the product for the user to finish purchasing.

06/2019