You have to install the VS 2019 Preview to resolve this issue.
Refer to this thread, IntelliSense does not work in client-side Blazor projects, for more information.
If the issue persists, make sure to install the .NET Core Preview 5. https://devblogs.microsoft.com/aspnet/asp-net-core-updates-in-net-core-3-0-preview-4/
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