To install a preview or release candidate (RC) package via NuGet in Visual Studio, you will need to follow these steps:

1. Open Visual Studio and go to the “Tools” menu. Select “NuGet Package Manager” and then choose “Package Manager Settings”.
2. In the settings window, select the “Package Sources” tab.
3. Click the “Plus” icon to add a new package source.
4. In the “Name” field, enter a name for the package source (e.g. “Preview Packages”).
5. In the “Source” field, enter the URL of the package source. For example, to install preview packages from the ASP.NET Core team, you could use the following URL: https://dotnet.myget.org/F/aspnetcore-ci-dev/api/v3/index.json
6. Click “Update” to save the new package source.
7. Go back to the Solution Explorer in Visual Studio and right-click on your project. Select “Manage NuGet Packages”.
8. In the NuGet Package Manager window, select the package source that you just added from the dropdown list.
9. Search for the package that you want to install, and then select the preview or RC version from the list of available versions.
10. Click “Install” to install the package.