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If you refer to a checkbox, you can do as follows:
HTML example:
<form action="">
<input type="checkbox" id="my_checkbox" value="Bike">Checkbox<br>
</form>
Javascript:
var myCheckbox = document.getElementById('my_checkbox');
myCheckbox.checked = true;
Accessing inside an iframe:
var myFrame = document.getElementById('my_frame');
var myCheckbox = myFrame.contentDocument.getElementById('my_checkbox');
myCheckbox.checked = true;
take the checkbox id, and mark the checked property as true.
– rray
example with jquery: http://answall.com/questions/49673/fun%C3%A7ao-para-marcar-desmar-checkbox-jquery
– Daniel Omine