To Activate Kaspersky Intenet Security you would need to have the activation code first. If you still do not have one you would be able to get the Kaspersky license code here.
Note: You would need to have Kaspersky Internet Security installed on your device (supported versions are: 2017,2018 and 2019).
Then folow below steps:
1- In the Licensing window, click License: … days remaining.
2- Click Enter activation code.
3- Enter the code from the message you have received after buying the license and click Activate.