Issue with Dictionary field. It is disabled in UiPath



I have created following class in c# Library, which is imported in UiPath as NuGet package.
InputTable with collection is disabled as shown in the attached screenshot.
Kindly let me know, how to enable this field so that user can click the “…” and fill the collection.

public class TableFields : CodeActivity
public Dictionary<string, Argument> InputTable { get; set; }

    public TableFields()
        InputTable = new Dictionary<string, Argument>();

    protected override void Execute(CodeActivityContext context)
        throw new NotImplementedException();


Custom Activity: Collection as Input Argument