When you install Tekla Structural Designer, you will be asked to select your license method from a range of options. These are fully explained in the "Tekla Engineering Software - Installation and Licensing Guide 2020" which can be viewed during the installation process by clicking the button shown below.

To install Tekla Structural Designer:
If you are upgrading an existing
Tekla Structural Designer version, you will also need to upgrade your local databases, see:
Install the upgrade.