Thanks for the reply. I am already using @Nobita.Kun's quick reply for guest addon. In that, guests can add replies to existing threads. In this feature that I am looking for, the guest will be creating a new thread in a specific sub forum that allows guest posting. I don't want to force them to become a register.
I have a parenting website. Many times parents who have kids with special needs do not want to sign up and they are sending us emails instead.
I tried allowing guests to post in one sub-forum. But many never noticed it. If this feature is available as a sidebar widget or as a block above the forum list, I think it will have better chances of getting used. Something that works for @xfrocks's widget frame work will be great.
If he releases a paid addon, I will be willing to buy it.