To get a new sub category associated with the product you will need to first have a recordset on the page that selects all of the sub categories from the sub categories table.
This will be the same for an update or insert page. Once you have this recordset in place you can add in a select list to the form. It will be a regular select list, make sure to give it a meaningful name. Next you will go to the property inspector and choose to make the options dynamic. You will select your new sub category recordset and set the values and labels accordingly. It should be the id for the value and the name for the label.
This will give you a select list for the sub category on your page. Once this in on the page and you can select from the various sub categories you will be able to incorporate that into your insert or update server behavior. There is one more step for an update page, you will need to make sure that the initial value is set equal to the sub category for the current product from the existing products recordset. This is done with the dynamic button in the property inspector for the select list.