You don't need the button if the user making a combo selection would be the point at which you'd generate the subform records. If there has to be some other interaction with the form that must happen first, then maybe a button. The combo/button would be on the main form. The main form would need a subform control. When you drag the subform control onto the main form's detail section, Access will prompt you to select the form that should go in there if you have "Use Control Wizards" selected in the Ribbon, which means you should design the subform query first, then the subform. When prompted, refer to the subform you have created. You can also base the subform on a table if the db design allows it, but I try to use queries as they're easily modified. If more than one table is involved in the subform records, you have to use a query anyway, not to mention that if you're going to use a form control for criteria input, it's far easier using a query.
The subform wizard will give a name to the subform control that (usually) is the same name as the subform you put in it. That's not a real good thing; I suggest you rename it according to the naming convention you have (I hope) adopted. The query criteria for the proper field should refer to the form control as in
Code:
Forms!frmYourFormName.YourControlName
Either the combo box AfterUpdate event (alternatively your button click event) should requery the sub form, which will automatically run the query.
Not enough info in your post to be able to say much more than that. At least you have a start - create the query, subform and add it to the main form. I don't see from your information that you need to worry about parent/child linking between main and subform. Perhaps you should research Access subform creation to bring yourself up to speed.
Access does have what's called a split form, which is an easier route for those without the knowledge required for subforms, but they certainly do have their limitations. IMHO, you should avoid them if you can.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.