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