Here is how to upload Facebook posts to Threads automatically, so that every time you post on Facebook, the post gets automatically posted on Threads (and any other social networks you select).
Using OneUp, you first connect your Facebook account and Threads account (and any other social network you want to autopost to):

Note that this only works with Facebook Pages (and not personal Facebook profiles).
Then choose “Cross-posting” from the Create A Post dropdown options:

Then choose Facebook as the “Post from” account, and choose Threads (and any other social network you want) as the “Post To” account:

That’s it! Now moving forward, whenever you post on Facebook, it will automatically post to your Threads account with 2 hours of the original post.
Or you can optionally choose to add a delay so that it doesnt post to Threads until after a few hours or days.
Here is a video showing how the autoposting process works in OneUp:
FAQ
Does this work with both image posts and video posts?
Yes, OneUp’s cross-posting works with both image and video posts. If you publish an image post to the Facebook Page, it will autopost to Threads. If you post a video post or Reel to the Facebook Page, it will autopost to Threads.
If I add music to my Facebook post, will that music also be added to the Threads post?
Yes, it will post the video with the music to Threads.
So this works even if I post to the Facebook account from the Facebook mobile app?
Yes, this works regardless of whether you post to Facebook from the Facebook mobile app, from facebook.com, or from a scheduling tool.
Does this work for autoposting from Facebook to multiple different Threads accounts at the same time?
Yes, OneUp’s crossposting feature allows you to autopost from Facebook to multiple Threads accounts at the same time.