You can copy the following code and place it in the control panel of your store by going to the settings then advanced settings then custom code and put the code in the header scripts
<!--Start of Zendesk Chat Script-->
<!--End of Zendesk Chat Script-->
The zendesk application is free for all plans ... You can install the application by going from the control panel to the applications and services .
On the next link you can learn more about the prices of plans with the applications attached with each one