Submit form: Assert that the DOM is completely rendered

I’d like to submit a form but add some hidden inputs first. The hidden inputs are added with a ng-repeat. Eventually they will be rendered, but how can I be sure that the DOM is already updated prior to triggering the submit event?

partial:

<form ng-submit="addValuesAndSubmit()">
  <input type="hidden" name="{{name}}" value="{{value}}" ng-repeat="(name, value) in order">
  <input type="submit">
</form>

controller:

$scope.addValuesAndSubmit = function() {
  $scope.order = { param1: 1, param2: 2 };
  // TODO: wait until form is rendered
  // there should be two <input type="hidden"> now
  // trigger submit action
};

As I understand it, a post-linking function for a directive could be used for this. Am I right or am I missing something obvious?

EDIT: The form data needs to be posted to an external website, redirecting the browser to the response. It’s a payment integration where I calculate an HMAC on the server, add it to the form as a hidden element and then post it to the payment provider. The implementation of $http.post() seems not to do that redirect but returns the response instead.