If you want to have a form where the check-boxes are pre-checked, you can set it up in the Form editor.
Step 1: Click the pencil icon to edit the Checkbox field:
Step 2: Insert values for each option. Note that the values for the options should be different:
Step 3: List these values in the Default value field (comma separated):
Result: The options on the form will appear as pre-selected:
