In order to find your marketing list ID, you first need to navigate to the marketing list via the 'Your Data' tab on the left-hand menu:

This opens all of the Marketing Lists in your account, from this page find the list you need the ID for by using the search at the top left of the screen and search for your Marketing List name:

From here, click on the Marketing List name to open the page up. The marketing list ID can be viewed within the browser URL after the 'marketing-list/' folder in the URL.

The ID appears as a long series of numbers and letters, for example, the highlighted text below would be the ID:
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If you need any further assistance, please reach out to the support team on the live chat or log a ticket via the Support Hub.