If GIT is installed on your system, skip this step, otherwise install. After installation, restart Unity and Unity Hub
Open "Window -> Package Manager"
Select "Packages: In Project" and click "add from git URL"
Enter the package address (e.g. https://github.com/jilleJr/Newtonsoft.Json-for-Unity.git#upm) and click "add"
After adding a package, it will appear in the general list and you can use it.