Access customParameters or any object(not defined as property in Activity class) from Activity class in QnAMaker-all features bot framework code

Question

I am using QnAmaker-all features C# dotnet sample code to work with. For my task I am passing some extra parameters from webchat UI and want to receive in activity context in the bot framework code. But it gives error like this :
error CS1061: 'object' does not contain a definition for 'customParam' and no accessible extension method 'customParam' accepting a first argument of type 'object' could be found (are you missing a using directive or an assembly reference?) [D:\local\Temp\zipdeploy\extracted\QnABot.csproj]

this is the bot code where i access the data from the Activity:

this is how I am passing data with name customParam from my webchat UI: