NB: Bear in mind that the default value you're changing in code will only affect the Control's default value and not affect the field's default value in the table. So for everytime you open the form, you must run that code.
You would use the TableDefs collection to change a field's property. In order to effectively do this you must have exclusive access to that table (i.e. the table must not be in use or open).
If this is going to be a regular occurence, then the TableDefs option wouldn't be suitable.