namespace MyGameProject;

public class ItemSlot : ButtonBase<Panel>
{
    // ...
}

Project.ExtraWidgetAssembliesAndNamespaces.Add( typeof( ItemSlot ).Assembly, new string[] { typeof( ItemSlot ).Namespace } );
Grid g = Project.LoadFromXml(File.ReadAllText(Path.Combine(Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location), "assets/inventory.xmmp")), MyraUtil.AssetManager).Root as Grid;
var slots = g.GetChildren().Where(w => w is ItemSlot).Cast<ItemSlot>().ToArray();
slots[0].Stack = new Items.ItemStack("dagger") { Quantity = 1 };