Backbone form - custom template for a field - not submitting the value

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SO i have a checkbox in my template which is (jade)

input(type='checkbox' name='flag' class='pure-checkbox onoffswitch-checkbox' id='myonoffswitch')

and i use this custom template form my flag field


schema:
    flag:
      template: require ('templates/onoff-switch.hbs')
    threshold:
      type: 'Text'
      title: 'Threshold($)'
      editorClass: 'pure-input-1-3'


but when i submit my form the flag value is not set, its empty. I hope i 
have to bind my value to the checkbox value but i am sure how.

i tried

 input(type='checkbox' name='flag' class='pure-checkbox onoffswitch-checkbox' id='myonoffswitch' checked="{{#if flag}}checked{{/if}}")


Any idea?