You can use the GoToControl action in the After Update event of the combo
box. Note that you would need two such actions -- the first to set focus on
the subform control (the control that holds the subform source object), and
the second to go to the control in the subform (you must reference this
control through the subform).
Action: GoToControl
Control Name: < name of subform control >
(e.g., MySubform)
Action: GoToControl
Control Name: <name of subform control >!<name of control on subform>
(e.g,. MySubform!MyControlName)