...
var
 DC : HDC;

...
dc:=GetDC(0); // The Handle of the Desktop
MoveToEX(dc,0,0,nil);  {Handle, X & Y for the new Coordinates
                        And the Rectangle of the wanted area}
LineTo(DC,300,500);
LineTo(DC,500,300);
...
ReleaseDC(0,DC);
...