I'm using interface for most of my stuff. I can't find a way to create an overload operator + that would allow me to perform an addition on any objects implementing the IPoint interface

Code

interface IPoint
{
    double X { get; set; }
    double Y { get; set; }
}

class Point : IPoint
{
   double X { get; set; }
   double Y { get; set; }

   //How and where do I create this operator/extension  ???
   public static IPoint operator + (IPoint a,IPoint b)
   {
     return Add(a,b);
   }

   public static IPoint Add(IPoint a,IPoint b)
   {
      return new Point { X = a.X + b.X, Y = a.Y + b.Y };
   } 
}

   //Dumb use case :
public class Test
{
   IPoint _currentLocation;

   public Test(IPoint initialLocation)
   {
     _currentLocation = intialLocation
   }
   public MoveOf(IPoint movement)
   {

      _currentLocation = _currentLocation + intialLocation;
     //Much cleaner/user-friendly than _currentLocation = Point.Add(_currentLocation,intialLocation); 
   }
}