Workspaces are not containers. A draft is not “in” a workspace, exactly. They are sets of filters and options you can apply to get different views of your drafts. More detail in the user guide article.
Most commonly, they are used with tags. So, a simple example, you might set up a “Personal” workspace, with a tag filter of “personal”. When you select that workspace, only drafts with the tag “personal” assigned are visible in the list.
So making a draft appear in that workspace is simply a matter of assigning the draft the tag “personal”.
Does that help clarify?