BindAsync.cs

public Task<ITriggerData> BindAsync(object value, ValueBindingContext context)
{
    if (value is SlackMessage)
    {
        var bindingData = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase)
        {
            {"data", value}
        };

        object argument;
        if (_parameter.ParameterType == typeof(string))
            argument = JsonConvert.SerializeObject(value, Formatting.Indented);
        else
            argument = value;

        IValueBinder valueBinder = new SlackMessageValueBinder(_parameter, argument);
        return Task.FromResult<ITriggerData>(new TriggerData(valueBinder, bindingData));
    }
    throw new Exception();
}