Hi 

I assume that you added a popup when clicking on the inventoryID, In the popup Add a Button as Add or OK (Refer to the screenshot). In the Add/OK button, Add the code to insert the related item load on the Soline.
 

Sample Code for the OK/Add button:

public PXAction<SOOrder> RelatedProductsOK;
         PXButton]
        ÂPXUIField(DisplayName = "Related Items")]
        protected virtual void relatedProductsOK()
        {         
          InventoryItemFilter.AskExt();

            if (InventoryItemFilter.Cache.Inserted.Cast<InventoryItemFilter>().Count() > 0)
            {
                if (InventoryItemFilter.AskExt() == WebDialogResult.OK)
                {
                    foreach (InventoryItemFilter invt in InventoryItemFilter.Cache.Inserted)
                    {
                          if (invt.Selected==true)
                        {
                            SOLine newline = PXCache<SOLine>.CreateCopy(Base.Transactions.Insert(new SOLine()));
                            newline.InventoryID = invt.InventoryID;
                            newline.OrderQty = 1;
                            newline.SiteID = invt.DfltSiteID;
                            Base.Transactions.Cache.Update(newline);

                            InventoryItemFilter.Cache.SetValueExt<InventoryItemFilter.selected>(invt, false);
                        }
                    }
                }
            } 

Thanks for the information.  Our developer had to do some additional work because the Related Items pop up screen was a little more complicated, but he was able to get it working.