Show the item name when selected the item id from lookup

On your table in the AOT, overwrite the modifiedfield method. For an example on how to overwrite, check the purchline.modifiedField method for example. Just add a case in the switch:

case fieldnum(YourTable, ItemId) this.itemName = inventtable::find(this.itemid).name(); break;

Unless really necessary, adding both the itemId and itemName is not needed, as you are de-normalizing the database at the point. It is probably better to add a display method for the itemname to your grid.