Hi and welcome to the forum!
It depends on how you wabt to do it. One approach would be to fill out the default config file such that each new backup is pre-filled with the options you like.
If you want to customize it further, you can make a custom UI page, based on the current S3 page. For this approach you need to hook into the setup.
To make it even more custom, you can create a new backend module that is based on the S3 module. With this approach you can fully hide the option to use anything else than your chosen server(s). You can provide a custom UI for it with the same approach as mentioned above. You can also remove the other backends if you only want to support your custom one.