[web2py] How to attach an onclick event to a custom form field?

[alex]

I have this custom form:

 {{=form.create_form.custom.begin}}
      {{=*form.create_form.custom.widget.field*}}
      {{=form.create_form.custom.submit}}
 {{=form.create_form.custom.end}}


how do I attach an event "onclick" to the widget 'field", to obtain this?

<input onclick="foo()" id="form_field" name="field" type="text" value="" />

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