To activate a student license you need to have a product code. If you bought the license your self you can find it in the mail you got.
First install Mathcad Prime. And the first time you start it it will automatically start the license wizard. If you already are using Mathcad Prime Express you can click on the Activate button to start the wizard:
Choose "Activate purchased license":
You will the see a log in screen where you need to log in with a PTC account. If you don't have an account you can create it here. That account can later be used to reach other resources on the PTC web:
The you should see a form where you can enter you product code:
You will then automatically get a license file sent to your machine and can start using all features.