Check out example codes for "bootstrap form input select". It will help you in understanding the concepts better.

Code Example 1

<div class="col-lg-6 col-md-6 col-12">
  <div class="input-group mb-3">
    <div class="input-group-prepend">
      <select class="form-control select2bs4" name="country_code" id="country_code" style="width: 100%">
        <option value="+91">+91</option>
        <option value="+351">+351</option>
        <option value="+1">+1</option>
      </select>
    </div>
    	<input type="text" name="user" class="form-control" placeholder="Email Or Mobile Number">
    </div>
</div>

Code Example 2

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Bootstrap Example</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css">
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
  <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/js/bootstrap.min.js"></script>
</head>
<body>

<div class="container">
  <h2>Form control: input</h2>
  <form>
    <div class="form-group">
      <label for="uname">Name:</label>
      <input type="text" class="form-control" id="uname">
    </div>
    <div class="form-group">
      <label for="upass">Password:</label>
      <input type="password" class="form-control" id="upass">
    </div>
    <button type="submit" class="btn btn-primary">Submit</button>
  </form>
</div>

</body>
</html>

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