Before optimizing for a conversion, please learn more about Optimizing for Conversions to ensure it is a right fit for your campaign.
To optimize your campaign for Custom Events, set your campaign's objective to "Conversions" on the Edit Campaign page and select your Custom Event. Next, go to the Edit Ad Set page of your eligible ad set. Under the Set a Bid section, select "Optimize for conversions" as your ad delivery option.
Set your target Cost per Action and click "Update" to save your changes. Please note that your target Cost per Action is not your final CPA.