如果选择的值为 SMTH，则跨度/隐藏输入

$( "#selectlist" ).change(function(event) {
  if (this.value == 1 || this.value == 2) {
    $(".form-control").show();
  } else {
    $(".form-control").hide();
  }
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form class="form-signup" action="admin_users.php" method="POST" id="payment-form">

                    <div class="well well-sm">
                   	<h3 class="form-signin-heading"> Create

                    <select id="selectlist" name="selectproduct" >
                        <option value="1">User</option>
                        <option value="2">Admin</option>
                        <option value="3">Smth else</option>
                        <option value="4">Smth else of else</option>

                      </select>
                      </h3>

                   	<div class="form-group">
                      <div class="col-xs-12">
                   		<input class="form-control" type="text" name="username" id="username" placeholder="İstifadəçi" value="first input" required autofocus>
    </div>
    <div> &nbsp </div>
                      <div class="col-xs-12">
                   		<input type="text" class="form-control" id="fname" name="fname" placeholder="Ad" value="second input" required>
    </div>
    <div> &nbsp </div>

    	            <input class='btn btn-primary' type='submit' name='addUser' value='Create user' />
                  </div>
                   </form>