The exact same thing happened to me last week when installing the fix adapter on my new hoover. In my haste, I mistook the foam seal for a "transport lock" and therefore didn't use it.
My electrician said it wasn't a problem at all, as the hoover is only used in dry indoor areas and so no moisture gets to the contacts. The adapter fits tightly even without a seal.
To be on the safe side, I will cover the small gap between the plug and the adapter with some insulating tape so that no dust or dirt can get in - and it provides minimal moisture protection.

The mousse is useless, it doesn't matter
I'd even go so far as to say that you shouldn't use it at all