Auto Populate a field in a form

Hi,

I have a form that is associated with a table, there is a
field in the table called ToCode (this is a service code,
i.e. 0548), this field is a combo box looking a table
that hold a list of these codes with a description (i.e.
0548 Financial & Technical Systems Support Unit.).

What happens is when I select a new entry and use the
ToCode combo box, this pulls up a list of codes with
there name and uses the first row (Code) and add it to
the associated table.

I want the description to auto add to the table using the
selected code from the combo box.

Does anybody know how this can be done within the form?