It's actually sort of the other way round. C# has hardcoded syntax for async/await. F#'s syntax for async/await is a fully-general user-accessible mechanism.
They're not so different in that regard. C# `await` can be adapted by making an awaitable and awaiter type[1], which isn't to dissimilar to how a computation expression in F# needs to implement methods `Bind`, `Return`, `Yield`, etc.
In both languages these patterns make up for the absence of typeclasses to express things like a functor, applicative, monad, comonad, etc.
