To post your weight-in or daily diary entry, you reply to the post's host.
You do this by scrolling down to where there is a 'reply' box you can use.
If you click the 'Write' button at the top of the screen, then you'll be making a new thread which isn't contributing to the weigh-in or daily diary thread.
(Don't beat yourself up if you do it, an administrator will likely explain how or refer you to this post. 😄)
Don't forget to reply to other posters in the thread. The Weigh-In and Daily Diary threads are most helpful when there's a conversation going on, commenting upon and offering suggestions to other posters, rather than a "post and run" job. 😄