Hi,
From your question i understand that you have a Multi-selection check box list and you also have a custom list which you are using it in the check box list. So to default only one value to be checked tick the Initialize box in the check box list configuration and in the Default value write something like ["busy"]
In my scenario i have a custom list with Label - Busy and Value - busy so i am using the value in the check box default value. Now my Busy check box is checked by default on page load.
If you are using a single check box then you just need to tick the Default Checked option.
Hope this helps.
Thanks,
Sushma