To do this, go to the design mode of the form page you would like to add the action in. You will find a list of your linked sheets under the Sheet Linking Tools menu, sorted beneath the type of linking. Click on the + icon next to the linked sheet you would like to add a new entry in.
You will be prompted to name the button that will be added to your action.
This will create an action button that will allow you to add a new entry to your subtable by filling out the full version of the form.