SOURCE

event_=document.getElementsByClassName('sub')[0].onclick=function(){
  document.getElementsByClassName('text')[0].value='hello world';
};

<div class="main">
  <div class="form">
  	<input type="text" value="" class="text"/>
    <input type="submit" value="start" class="sub"/>
  </div>
</div>

body,html {
  height: 100%;
  width: 100%;
  margin: 0;
  padding: 0;
}
.main{
  padding:0px;
  width:300px;
  height:100px;
  background-color:#efefef;
  position:relative;
  margin:0px auto;
}
.form{
  text-align:center;
  position:absolute;
  width:170px;
  height:50px;
  top:50%;
  left:50%;
  margin-left:-85px;
  margin-top:-25px;
}