Sep up pre-order for each variants

All of the steps to manage:

1. From the admin page of the app, click Pre-order
2. Click on Single product tab
3. Select any particular product which contains variants that you want to pre-order
4. Disable the checkbox Same pre-order settings for all variants
5. Select the variant that you want to add pre-order
6. Set up the pre-order for each variant as regular