Hi @Lennonsworld,
Thanks for getting in touch on this!
I double checked for you, and that's not actually a feature on Dropbox Plus, which is why you don't see the option.
Sorry for any confusion!
If you scroll down to the "Compare all features" section here, you can verify which plans have this option, and what other features are available: https://www.dropbox.com/plans
You can use a third-party app to add a watermark before you upload to your Dropbox Plus account, or upgrading to Professional would bring that in for you as a built in feature.
Hope that helps!