<div class="panel panel-default">
  <div class="panel-heading"><h3 class="panel-title">Add Record</h3></div>
  <div class="panel-body">
    <form action="/add" method="post">
      <div class="form-group">
        <label for="title">Title</label>
        <input class="form-control" type="text" name="title" id="title">
      </div>
      <div class="form-group">
        <label for="url">URL</label>
        <input class="form-control" type="url" name="url" id="url">
      </div>
      <div class="form-group">
        <label for="username">Username</label>
        <input class="form-control" type="text" name="username" id="username" inputmode="verbatim">
      </div>
      <div class="form-group">
        <label for="other">Other</label>
        <textarea class="form-control" name="other" id="other"></textarea>
      </div>
      <button class="btn btn-default" type="submit">Add</button>
    </form>
  </div>
</div>

<div class="panel panel-default">
  <div class="panel-heading"><h3 class="panel-title">Add Record</h3></div>
  <div class="panel-body">
    <form action="{{url_for('add_record')}}" method="post">
      <div class="form-group">
        <label for="title">Title</label>
        <input class="form-control" type="text" name="title" id="title">
      </div>
      <div class="form-group">
        <label for="url">URL</label>
        <input class="form-control" type="url" name="url" id="url">
      </div>
      <div class="form-group">
        <label for="username">Username</label>
        <input class="form-control" type="text" name="username" id="username" inputmode="verbatim">
      </div>
      <div class="form-group">
        <label for="other">Other</label>
        <textarea class="form-control" name="other" id="other"></textarea>
      </div>
      <button class="btn btn-default" type="submit">Add</button>
    </form>
  </div>
</div>

Use url_for in add form
Use url_for in add form
