Hi Arif,
The Write-In Products names field is productdescription.

Here is my test.
I download the word template and edit it, only insert a single field productdescription. Then upload it into D365.

In an Opportunity, I create a Write-In Product, set its name to "My first write-in product".

Then I download the uploaded word template in the Opportunity form, it shows the Write-In Products name.

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Since the name field of Existing Prodcuts and Write-In Products are not the same, if you want to display both the name of Existing Prodcuts and Write-In Products in only one column in the word template, then we can create a new calculated field to store the names of both of them.
When the product exists, the value of the new field is productname, when the product is written, the value of the new field is productdescription.
Finally insert the new calculated field into the word template to represent the name of product, no matter it is Existing Prodcuts or Write-In Products.

