There is no official Microsoft text file that activates Office 2016. The "txt" method is a euphemism for unofficial, often illegal, volume activation hacks.

Replace <product_key> with your actual product key and <txt_file> with the path to your text file containing the product key.

Activating using a "txt" method typically refers to using a batch script ( .cmd or .bat ) to connect to a Key Management Service (KMS) server. This method is often used to activate volume-licensed versions of Office without a standard retail product key. Guide to Activation via TXT/Batch Script