If you are logged into your App Store connect account, go to "My Apps" and choose the desired App.

On the next screen, select the "Features" tab and then on the "+" icon beside "In-App-Purchase".
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Select the type of Subscription you would like to create in the pop up window (this example will continue creating an "Auto renewable Subscription") and continue by clicking on the "Create" button:
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​Enter the reference name and Product ID for your subscription on the next pop up and continue by clicking on the "Next" button:

You will be forwarded to the next screen where the settings of your Subscription can be edited:
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The following entries will need to be filled out to finish setting up your subscription: 

Save your entry by clicking on the Save button in the top right corner.