Start by going to your Event Page. Next, go to your Event Settings step and scroll down until you see Financial Information. Look for the Accepting Subscription toggle and click it so it says YES, then make sure you choose the Subscription Events as well. 

After this, move into the Event Setup step and click to edit your ticket levels. Once you click to edit, turn the Eligible for Subscription toggle to YES and select your Subscription Event in the dropdown menu. Then, don't forget to Update your ticket level at the bottom. 

Once this is done, attendees will be able to apply their subscription code to your event. Here is an example of what this will look like on the event page.