Presetting Checkboxes and Radio Buttons in Prefilled Forms
When using the prefilled forms feature, and you want checkboxes and radio buttons to be preset based on known information about the respondent, be sure the corresponding <input> tags in your form document include a value attribute.
Checkbox Example
<p><input type="checkbox" name="HTML" value="yes">I prefer HTML messages.</p>
Note: The value attribute ensures that the data is transmitted to the HTML page during prefilling.