Making multiple checkboxes initially checked on a form

I'm creating a form where one of the fields is a multi-select checkbox. I would like to have all checkboxes initially set as ticked when the person first comes onto the page. I can do this with a single checkbox (the Initially Checked option), but there doesn't seem to be an option to do it for multiple checkboxes.

Does anyone know a way I can make this happen?

Thanks

Natali