The easiest way to display a custom property declared for the point is to use the tooltip pointFormat.
Inside that property, you can decide which value from point should be shown in the tooltip. It should be declared as an HTML string and properties from point to show should be in curly braces.

tooltip: { 
      pointFormat: 
              'The value for <b>{point.x}</b> is <b>{point.y}</b><br>{point.custom.extraInformation}' 
},

A recommended way of declaring that extra information is to use the custom property.

See that in the example.