Altium Designer board outline and polygon
Your question is not entirely clear. My interpretation is that you want the polygon to automatically follow the board outline. 

If this is the case, create a design rule in the "BoardOutlineClearance" section under the "Manufacturing" heading. Give it a distance like 0.5mm (how far away from the board outline you want your pour to lay). 



Once applied, when drawing a polygon draw the rectangle around the entire board, and only the inside will fill in, leaving a gap of the predetermined width between the pour and the board outline.