On a form a hve a combo box, when the user selects an item from the list it puts a code into a text next to it.
I have put a formula in the text box to do this.
When I go to the table where the records are saved there is nothing in the code field.
Have can I get the text box to save the code into the table

Normally you do not save a calculation but do the calculation as you need it. You can probably do your calculation in the After Update event of the Combo box and then put in a line of code similar to this:

Me.TextBoxBoundToTable = Me.comboboxName * Calculations

I have used the multiply symbol but you know how to do the calculation.