Click on the External Script Editor dropdown and select Microsoft Visual Studio.

To set your default script editor manually:

For more information, see Downloading and installing Editors and modules with the Unity Hub. You can install Visual Studio as a module into an existing Unity installation. Unity automatically uses Visual Studio as its default script editor if Visual Studio is installed when you install Unity, or if you install Visual Studio as part of the Unity installation process. The Visual Studio Editor in the Package Manager Window Set Visual Studio as your default script editor If Visual Studio is installed at the time you install Unity, then Unity uses Visual Studio to open and edit scripts A piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like.

This package is pre-installed when you install Unity. Unity integrates with Microsoft Visual Studio through the Code Editor Package for Visual Studio.

Unity notepad++ editor