HTML

<div class="wrapper">
  
  <button class="form-control" id="keygen">Generate API Key</button>
  
  <input class="form-control" id="apikey" type="text" value="" placeholder="Click on the button to generate a new API key ..."  />
  
</div>

CSS

body {
  text-align: center;
  background: #f1f1f1;
}

.wrapper {
  padding-top:60px;
}

button.form-control {
  background: #f7f7f7 none repeat scroll 0 0;
  border-color: #ccc;
  box-shadow: 0 1px 0 #ccc;
  color: #555;
  vertical-align: top;
  border-radius: 3px;
  border-style: solid;
  border-width: 1px;
  box-sizing: border-box;
  cursor: pointer;
  display: inline-block;
  font-size: 13px;
  height: 28px;
  line-height: 26px;
  margin: 0;
  padding: 0 10px 1px;
  text-decoration: none;
  white-space: nowrap;
}

input.form-control {
  background-color: #fff;
  border: 1px solid #ddd;
  box-shadow: 0 1px 2px rgba(0, 0, 0, 0.07) inset;
  color: #32373c;
  outline: 0 none;
  transition: border-color 50ms ease-in-out 0s;
  margin: 1px;
  padding: 3px 5px;
  border-radius: 0;
  font-size: 14px;
  font-family: inherit;
  font-weight: inherit;
  box-sizing: border-box;
  color: #444;
  font-family: "Open Sans",sans-serif;
  line-height: 1.4em;
  width: 310px;
}

JS

/**
 * Function to produce UUID.
 * See: http://stackoverflow.com/a/8809472
 */
function generateUUID()
{
	var d = new Date().getTime();
	
	if( window.performance && typeof window.performance.now === "function" )
	{
		d += performance.now();
	}
	
	var uuid = 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c)
	{
		var r = (d + Math.random()*16)%16 | 0;
		d = Math.floor(d/16);
		return (c=='x' ? r : (r&0x3|0x8)).toString(16);
	});

return uuid;
}

/**
 * Generate new key and insert into input value
 */
$( '#keygen' ).on('click',function()
{
	$( '#apikey' ).val( generateUUID() );
});