Do it like this ... your textarea must exists but you hide/show it depending on a condition.

 



.. and write a function to change the visibility of myDIV,
this function must be executed when you click your button.

function myFunction()
{
  var myDIV = document.getElementById("myDIV");
  if( myCondition )
  {
     myDIV.style.visibility='visible';
  } 
  else
  {
      myDIV.style.visibility='hidden';
  }
}


. . . . . . .