Write-in Products are a great way to add a line item to a Quote when the actual product doesn’t exist in the product catalogue. For example, if you wanted to quote an estimate for a construction project, you could do that using a Write-in Product.
In order get the Write-In Product option, you need to enable the privilege in the Security Role for the user. The privilege is called Override Quote Pricing and is found in the Sales tab of the security role:
Set the privilege accordingly, and the Write-in Product option will then appear in the Quote Products subgrid of your Quote form: