This is from the XenForo forum help site. Don’t know if it will work for you since it appears drafts are only saved for 24 hours:
“Just create the new thread as normal and type your post, then save the draft (the automatic draft saving will likely have saved it but this makes sure that it has saved everything you have typed). Later when you want to retrieve the post, simply visit the same forum you made the draft in, click on Post New Thread and the draft thread will be retrieved.”
Just tested the process and it worked for me. Set up a test post in this forum, saved the draft, then left the forum. When I came back and hit the new post button, my test post reappeared.