Situation:
The form is at <https://example.com>
If user write "demo" in input then submit, they will be redirected to
<https://demo.example.com> where "demo" is from the value they've putted before.

How to do that?

My code:

```html
<form class="form-inline" action="http://example.com" method="GET">
  <div class="form-group">
    <label class="sr-only" for="exampleInputAmount">Login</label>
    <div class="input-group">
      <div class="input-group-addon">https://</div>
      <input type="text" class="form-control" id="exampleInputAmount" placeholder="username" value="">
      <div class="input-group-addon">.example.com</div>
    </div>
  </div>
  <button type="submit" class="btn btn-primary">Login</button>
</form>
```