Right now the Grammarly Browser Extension (https://chrome.google.com/webstore/detail/grammarly-for-chrome/kbfnbcaeplbcioakkpcpgfkobkghlhen) doesn't work properly when composing a thread.
It will show suggestions, but you can't apply them. One workaround is to compose documents in another composer (like Google Docs) that supports Grammarly and then copy it into Threads to post, but this is cumbersome and can often lead to formatting issues.