Hope it helps!
If you only need a basic license (for a smaller app it is fine) you open the project in the developer portal, go to settings, and see an “Upgrade Your Plan” button on the top right. That will be $12/user/month. I believe it is a great deal for smaller apps, it includes monitoring, hosting, everything... If you need a bigger plan, like a private cloud then follow the steps on the link above sent by Srilatha
Hi, Hoang Pham Dinh
To buy a license you will simply have to go through the upgrade your plan steps on the Mendix platform. But to start out with if you have a small application that you only want to test out first you can simply publish it as a free app to start by opening the app in studio pro and pressing the publish button located in the top right corner.
Then if you want to get a license for your application simply go to the upgrade your plan button in your application file on the Mendix platform. This will then giude you to get a license and how to implement it.