If you aren't compelled to use a subform, the Combo Box wizard has an option
to use the Combo Box to search and display the selected record on the main
Form.
If you really want a Subform, you can set the MasterLinkFields to refer to
the Combo you use to Search and the ChildLinkFields to the corresponding
Field in the Data displayed in the Form embedded in the Subform Control, and
it'll be more-or-less automatic.