Interval + Check Element.js

 window.myInterval = setInterval(function() { //sets an interval
  
  	if ($('someElement').length) { //checks if element exists. In this case, we are looking for the button element.
  
    		//add the variation code either custom written OR rom Optimizely here
    
    	clearInterval(window.myInterval); //after element has been found, clear the interval so the page doesn't load continuously
 
  }
}, 50 )//interval timing in milliseconds;