I am a beginner 
How can I assign a draft to a specific workspace?
Start by reading the Workspaces article in the User Guide. Workspaces are not containers, like folders, they are filters. A draft is not “in” a workspace (in could, actually, appear in many workspaces).
The simplest case to start with is defining a workspace that filters by a single tag, like say “project-name”, and a draft will appear in that workspace if you assign it the tag “project-name”