Manually setup a NuGet package

I wanted to use Window.Forms, in particular charts within .Net 5.Therefore, I manually downloaded "system.windows.forms.datavisualization.1.0.0-prerelease.20110.1.nupkg" from the NuGet website, which is still a prerelease.I applied Tools > NuGet Package Manager > Package Manager Settings > Package Sources to setup the new path, where I saved the nupkg file.Unfortunately, trying to finally install the package via the Solution explorer > Managing NuGet Packages > Browse, the nupkg file will not be found.