Update Subform based on Form Combo Box

I am attempting to update a subform based on my combo box selection from the main form. I have one combo box that selects my entity and then another combo box that selects the codes available for that entity. I got those to work on the main form. I now want to add a subform that filters based on the code selected to show information that comes from the same table used in the main form. Right now all codes associated with the table show up and I am lost on how to show nothing when nothing from the combo boxes is selected and then updates to show data once selections have been made.