<div class="header">
   <div class="header__video-wrapp">
      <div class="header__video-box">
         <video class="header__video" id='header__video'  loop autoplay>
           <source src="https://s3-us-west-2.amazonaws.com/coverr/mp4/Winter-Grass.mp4" type="video/mp4">
          <source src="https://s3-us-west-2.amazonaws.com/coverr/mp4/Winter-Grass.mp4" type="video/webm">
      </video>
    </div>
</div>
 <div class='header__video-play'   onclick='document.getElementById("header__video").play();'>play</div>

<video id="video">...</video>

<style>
#video {
position: absolute;
width: 100%;
height: 100%;
z-index: -1;
}
<style>