If you know what the default should be, and it will be available from the row source, you can just set the combo's default property to the value you are going to want from design view. Or you can use code in the form's On Load event.
What you're actually doing with the Value from this Combobox needs to be considered.
Is the Combobox Bound to a Field in the Form's underlying Table or Query, i.e. is there a Field Name in the Combobox Control Source Property? The simplest way, in this scenario, would be using the Default Value in the Property pane, as Bob suggested.
If you want to do this through code for this scenario you could also use something like
Code:
Private Sub Form_Current()
If Me.NewRecord Then
Me.ComboBoxName = "Default Choice"
End If
End Sub
If, on the other hand, this Combobox is Unbound, and you plan on using the Value selected for something else, such as assigning it to a Textbox or using it as part of a calculation, that would be different. We frequently do things like this using the Combobox AfterUpdate event. The problem here is that when the Value is selected using code or thru the Default Value Property, the AfterUpdate event doesn't execute! You have to explicitly Call the AfterUpdate event.
In this scenario I'd use
Code:
Private Sub Form_Current()
If Me.NewRecord Then
Me.ComboBoxName = "Default Choice"
Call ComboBoxName_AfterUpdate()
End If
End Sub
If you subsequently elect to select another Value from the Combobox, using your mouse and the Combobox, its AfterUpdate event will be run again, based on the new Value.