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Bootstrap class: .segmented buttons

<div class="input-group mb-3">
  <div class="input-group-prepend">
    <button type="button" class="btn btn-primary">Action</button>
    <button type="button" class="btn btn-primary dropdown-toggle dropdown-toggle-split" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
      <span class="sr-only">Toggle Dropdown</span>
    </button>
    <div class="dropdown-menu">
      <a class="dropdown-item" href="#">Action</a>
      <a class="dropdown-item" href="#">Another action</a>
    </div>
  </div>
  <input type="text" class="form-control" aria-label="Text input with segmented dropdown button">
</div>

Preview