Net Core Add External Dll Reference

LibUser.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
  <Reference Include="Libext">
    <HintPath>Libext.dll</HintPath>
  </Reference>
</ItemGroup>

  <ItemGroup>
    <Content Include="dll\Libext.dll" Link="Libext.dll" CopyToOutputDirectory="PreserveNewest">
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </Content>
  </ItemGroup>

</Project>

Srt.cs

namespace LibExt
{
    public static class Str
    {
        public static string Capital(this string text)
        {
            return text[0].ToString().ToUpper() + text.Substring(1).ToLower();
        }
    }
}