Found an answer on Stack Overflow. You just have to change code to this:
public void InjectScriptResource(dynamic window)
{
var windowEx = (IExpando)window;
if (windowEx.GetProperty("signJson", BindingFlags.Default) == null)
{
// windowEx.AddProperty("signJson");
PropertyInfo p = windowEx.AddProperty("signJson");
// window.signJson = this;
p.SetValue(windowEx, this);
}
}