Do you want to create draft messages in Mastodon that you can send later?
There's no official feature for this, but if you're using the website or certain Mastodon apps you can do it unofficially:
1. Write your draft message but don't post it yet. Also don't @ anyone in it yet.
2. Set the visibility to "mentioned people only" or "direct"
3. Post it, click on "..." and bookmark it
You'll then be able to treat your bookmarks folder as a drafts folder. Only you can see these drafts because they are visible to "mentioned people only" and you didn't @ anyone.
When you want to actually post a draft:
4. Click "..." on the post and select "delete & re-draft"
5. Finish writing the message and feel free to @ whomever you want
6. Set the visibility to public (or whatever you want it to be), and post the message