If you have lost your original product key, your best option is to contact Microsoft Support to assist in recovering your digital license.
Unlike traditional desktop software that requires a 25-character alphanumeric product key, Microsoft Office apps under version 18.1903 generally rely on digital licensing models. 1. Device Bundling (OEM Licenses) microsoft office version 18.1903 product key
The number "1903" in the technical version coincidentally matches Windows 10 version 1903 (a major Windows update released in May 2019). However, Windows 10 1903 and Office version 18.1903 are separate entities. Some documentation and guides discuss using Office 2016 on Windows 10 version 1903, but the numbers are not directly connected beyond both originating from the same general time period. If you have lost your original product key,