There is no singular "Ymv8x" key. If a specific full key ends in Ymv8x , that specific key is valid only for the license it was sold with. You cannot simply "guess" the first 20 characters. The chances of successfully reverse-engineering a product key by knowing the last five characters are astronomically low.
With Microsoft 365, you pay
In 2025, over 95% of product keys ending in YMV8X will fail online activation because Microsoft has either retired the activation servers for Office 2010 or explicitly blocked the key. Microsoft Office Product Key Ending With Ymv8x