Yes, your discounts are compatible with TryNow orders. If a discount is applied automatically or applied via a promo code, the reduced total will be what the shopper is authorized for at checkout.
Note that buy X get Y discounts aren't supported by Shopify.
For a full list of requirements and restrictions, please see here.
Because the cart value of a Try Before You Buy order is $0, shoppers tend to use discount codes less frequently. While TryNow is compatible with discounts, many brands use TryNow to reduce the use of discount codes to drive more full priced sales.