Enter the desired centre-point in the main search field. You can enter a city/town name, street address, zip/postal code, or a combination of these. The geocoding service that will convert your search text to a set of coordinates may not recognize what you type in at first, so you may need to add or subtract pieces of information until it can find a matching location.
You can also enter a GC code here, but keep in mind that it will only be using that GC code's location as the centre-point for the search.
NOTE: Remember, the search tool is limited to a maximum distance of 30 miles/50 kilometres when searching around a point.