1. Go to Products & Forms.
2. On the Forms tab, click edit next to the relevant form.
3. On the Products tab, either add an existing product or create a new product.
4. Click the drop down arrow next to the product name, and select Mark as Mandatory next to the product. Click Save.
The product now displays as mandatory (the word will be displayed against the product).
5. To move the product back to optional, click Unmark as Mandatory.