Previously, there were a number of extra manual steps needed to add new customer / items to the demand plan. Now, it is as simple as adding them to the demand plan upload file as documented in the article Downloading, Editing, and Uploading a Demand Plan or in the article Uploading a User Defined Forecast / Demand Plan (S&OP Platform). Please refer to these articles to upload new item / customer demand plans.
In all cases, the condition is that there is no history available for DemandCaster to automatically create a demand plan. In addition, the customer and item must already be loaded in DemandCaster either from your ERP (preferred) or by creating in data maintenance.