Add -1 to existing text in a text box

[JeffreyDavid]

I have a sales order form with a text box containing the sales order number. At the bottom of the form I have a button to open a second instance of the sales order form. On the second instance of the sales order form, I have a locked text box which carries over the original sales order number. But on the second form, I need to add a -1 to the end of the sales order number.
Any ideas?

 

[WayneRyan]

Jeffrey,

Not sure "when" you want to assign the number, but
the proper reference to the main form is:

Me.NewField = Forms![YourMainForm]![ThatField] & "-1"

Wayne