How create Forum Thread with redirection to Forum create Thread Page with the passed body and subject props?

Sorry, but I'm not quite sure I follow. Can you not use the existing Forum - Create/Edit Post widget to create a thread? What are you trying to do?

Forum - Create/Edit Post - I'm using create API. But it's just created thread. I want to redirect to the /adddpost and I need autocomplete fields from the previous page form.

So it's a new, custom, widget similar to the Create/Edit Post, but instead of actually using the create API, it just collects the fields and then redirects to the existing Create/Edit Post page instead with it pre-filled? This is possible, but it would require customizing the existing Create/Edit widget to look for the passed content, likely via form post since it could be too long for a query string.

It's possible I'm still misunderstanding. And if I'm not, I still don't follow the use case for this over using the existing widget - potentially customized to work better outside of a contextual forum.