To pre-fill the checkbox question type via URL parameters, you can either pre-fill a single-choice or multiple-choice option. To select multiple choices, you can separate each of them with a comma. The values are case-sensitive, so please make sure to enter the correct option specified while building the question. The field code can be used in URL parameters, as shown below.
https://example.neetocal.com/sample-meeting?checkbox=Website,Ads