/**
 * pluginId - the numeric identifier of the checkbox plugin
 *   you are opting into
 */
_shopmsg('confirmOptin', pluginId);

If you are using an inline checkbox plugin, you will need to invoke a confirmation when your visitor opts in to messaging. After ensuring that your site complies with Facebook's policies you can confirmOptin in response to a click or explicit user action.

var myButton = document.getElementById('myButton');
myButton.onclick = function() {
  // opt-in the checkbox with id=56234 when visitor clicks #myButton
  _shopmsg('confirmOptin', 56234);
}