//creating a button and linking it to bClicked()
document.onload=()=>{
  let b = documentbody.createElement("button");
  b.onclick = bClicked;
}

function bClicked(){
  //yourID is the id of element that you want to fullscreen 
  document.getElementByID("yourID").requestFullscreen();
}

//to exit fullscreen
document.exitFullscreen();