Set a value when a widget come up

[admin]

You can use this example. Set Additional classes to widget-open for the element that opens the widget. #widget-2 is widget element id, change it and group address as needed.
Keep in mind that it won't work when transition animation is enabled.

Code:

$(function() {
  $('.widget-open').on('vclick', function() {
    var el = $('#widget-2');
    if (el.is(':visible')) {
      grp.write('1/1/1', 123);
    }
  });
});

There will be widget show/hide events added in RC2 firmware that will work correctly with animation:

Code:

$(function() {
  $('#widget-2').on('widget-show', function() {
    grp.write('1/1/1', true);
  });
  
  $('#widget-2').on('widget-hide', function() {
    grp.write('1/1/1', false);
  });
});