I figured this out The Constant Contact user interface is based on AJAX calls, so when you click on your lists, it makes an asynchronous call to get the lists. Included in this information is the ID you need. Go to your Contacts tab, Press F12, go to Network, And then press the "Email Lists" link. You'll see some new network requests, one of which ends with "lists". If you look into this request, it includes the seq_id, the number you need, for each list which is shown on the page. YAY!
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