Here are the steps to validate your plugin license.
Step 1: Getting the License Key
After you have successfully purchased the Email Customiser Plugin, please go to My Downloads page from Code Canyon.
Here is the step by step instruction :
Log into your Envato Market account.
Hover the mouse over your username at the top of the screen.
Click ‘Downloads’ from the drop down menu.`
Click ‘License certificate & purchase code’ (available as PDF or text file).
Here is an example of a PDF License certificate and purchase code:
This is a sample screenshot :

Step 2: Validating the License Key
Click on Settings icon

In the general tab, enter the License key.

Note :
Both the terms "License key" and "Purchase Code" refer to the same context.
The Validation of License key is used for automatic updates and support.
If your License key is not validated, please check if you have renewed your support.
Still unclear ?
Please submit a support request. We are always happy to assist you :)