How to keep the previous value before overwriting the textbox?

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C#

WinForms

Hi,
I'm trying to make a Form that will contain a TextBox into which I can write a value. All of this would be fine, but I have a problem with entering a value in the TextBox that does not appear in the List, the previous value would return to the TextBox.

What I have tried:

  private void elementTextBox_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            var elementString = elementTextBox.Text;
            elementTextBox_PreviewKeyDown(sender, e, elementString);//here is error Argument 2: cannot convert from 'System.Windows.Input.MouseButtonEventArgs' to 'System.Windows.Input.KeyEventArgs'	

        }
private async void elementTextBox_PreviewKeyDown(object sender, KeyEventArgs e,string element)
        {
            var myListOids = selectedOIDs.ToList();
            var myListOfElement = listOfElementNumber.ToList();
            DictionaryObjectId(myListOids, myListOfElement);
            var scaleString = scaleTextBox.Text;
            var scale = Int32.Parse(scaleString);
            var elementString = elementTextBox.Text;
            if (e.Key == Key.Return)
            {
                if (myListOfElement.Contains(elementString))
                {
                    var oidByElement = dictionaryOid.FirstOrDefault(pair => pair.Key == elementString).Value;
                    objectIdTextBox.Text = oidByElement.ToString();
                    await ScaleTask(scale, oidByElement);
                }
                else
                {
                    MessageBox.Show("Inadequate value!");
                    elementTextBox.Text = element;
                }
            }
        }

I have no idea how to pass that value, I thought (as it is in the code) that it could work when clicking into the field that it would take the value, but I don't know how to pass other values ​​to the method elementTextBox_PreviewKeyDown (object sender, KeyEventArgs e, string element )

I will be happy for any advice or idea.

David