you can define the default value of that parameter to be null, so that then you can check if that parameter is == null, so with your example it would be

  class Program
        {
            static void Main(string[] args)
            {
               var c= cc(5);
            }
            public static int cc(int a, int? b = null)
            {
              int c=0;
            if(b is not null)
            {
              c = a * b;
            }
            else
            {
              c =a*a;    
              return c;
            }
        }