Selection.End (Word)

Returns or sets the ending character position of a selection.

If this property is set to a value smaller than the Start property, the Start property is set to the same value (that is, the Start and End property are equal). The Selection object has a starting position and an ending position. The ending position is the point farthest away from the beginning of the story. This property returns the ending character position relative to the beginning of the story. The main document story (wdMainTextStory) begins with character position 0 (zero). You can change the size of a selection by setting this property.

pos = Selection.End 
Set myRange = ActiveDocument.Range(Start:=pos, End:=pos) 
ActiveDocument.Fields.Add Range:=myRange, Type:=wdFieldAuthor