How do I check all boxes by default in a multiple selection list box?

So I'm fairly new to both infopath and C#. I'm trying to get all the boxes checked in the multiple selection list by default whenever the data is imported to the form from a data connection. For example, whenever a field, ID, is filled, another field, Hours, gets populated in a multiple selection list from a data connection with all the boxes already checked. I've tried messing around with the rules, but to no avail. With my limited C# coding, I've been unable to come up with an answer as well. Any suggestions or hints would be greatly appreciated. Thanks!