how to set an option for form->input( 'multiple'=>'checkbox')

i plan to set a checkbox with selected option in my form.
but i am unable to show my checkbox content in the form, i cant see any value instead of just a box for me to select.

how to show value while i using checkbox? i able to show my value while i using select.
this is in a HABTM model. any hints?

here is my selection code.

input('User',array('label'
=> 'Select Related Potential',
'multiple'=>'checkbox',
//'options' => $users,

'legend'=>$users,

//'value'=>$users,

//'id'=>$ownUserId,

'default'=>$ownUserId,
'style'=>'width:200px;height:100px',
'selected' => $ownUserId, )); ?>