HTML

bootstrap 5 input group

<div class="input-group mb-3">
  <input type="text" class="form-control" placeholder="Recipient's username" aria-label="Recipient's username" aria-describedby="button-addon2">
  <button class="btn btn-outline-secondary" type="button" id="button-addon2">Button</button>
</div>

bootstrap 5 form input group

<div class="input-group mb-3">
  <span class="input-group-text" id="basic-addon1">@</span>
  <input type="text" class="form-control" placeholder="Username" aria-label="Username" aria-describedby="basic-addon1">
</div>