That ebay link isn't for the new 2018 Seamaster 300M -- it's for the previous generation. It likely won't fit. Your best bet is to go by a boutique or AD and order one from there. You might be surprised that it is cheaper than ones for sale on E-bay -- I have found since Omega has limited distributions of parts, sellers are jacking up the prices.
Case in point -- you can buy an Omega NATO from the on-line store for $160 + your state's sales tax. I see them for over $200 on E-bay all the time.