how to align two input class on the same line?

  html, php

I want to align two input classes next to each other with some space in between them like the following image.
enter image description here

However, my current output is like this:
enter image description here

The html code is

<div class="form-group">
    <div class="row">
        <label  for="form-workphoneNumber" class="col-sm-4 col-xs-4 control-label">input</label>
        <div id="box" class="col-sm-8 col-xs-8">
            <input type="text" class="form-control" name="workphoneNumber" placeholder="enter">
            <input type="button" class="btn btn-success" value="add" onclick="add_textbox()">

Source: Ask PHP