I think you simply need to insert in your drawing one of your parts that has your custom properties and then you have access to these properties via the "linked to property" command
It seems to me, as Vspamens says, that you must first of all insert your piece into your drawing, otherwise how can your new plan have a link with an undefined part. On the other hand, you can make custom properties for drawing whose extension is .drwprp, while for a part it's .prtprp.
@a.leblanc: No, SW doesn't need to have a file to bind a property. The fields will be empty in the template and in case of editing the basemap with no 3D model linked the fields will appear in plain text in the annotation ($PRPSHEET:{xxx}).