Entering your Target ACoS in the "Optimize" tab of the Amazon Advertising feature, is especially useful to automate your Campaigns.
Go to Sponsored Products Optimize.
Click on the Ad Group Tab.
Enter your Target ACoS manually or by upload. If you want some advice regarding how high or low you should input your Target ACoS based on your strategy check out this article.
P.S If you are having troubles uploading your Target ACoS check out this article.