How to add forecast manually for new items in Demand Management

Summary:

We have added a new item to the planning catalog used in demand management. This item does not have any shipment history. I have created a custom measure to add marketing forecast for such items. But I am not able to add the forecast in directly in the UI, as I do not see the new item part of any tables.

kindly suggest what should be best approach to load forecast for new items and as well as new customers.

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