document.cookie supports setting it, so Async Cookies should support as well. Specifically, the following assignments will be reflected in the Cookies view in Dev Tools.

document.cookie="sfoo=sbar; SameSite=strict";
document.cookie="lfoo=lbar; SameSite=lax";

Spec bugs:
https://github.com/WICG/cookie-store/issues/36
https://github.com/WICG/cookie-store/issues/38

We'll add a sameSite option to cookieStore.set() and make sure we reject if the option conflicts with the cookie prefix (__Host- implies sameSite=strict). We'll expose a sameSite attribute on cookie dictionaries returned by cookieStore.get() / cookieStore.getAll() and in change events.