In your .csproj file, add your dependency as a PackageReference in an ItemGroup, then run dotnet restore or nuget restore. Example:
<ItemGroup>
<Reference Include="System" />
<Reference Include="System.Xml" />
<Reference Include="System.Core" />
<Reference Include="Xamarin.iOS" />
<PackageReference Include="Realm" Version="2.1.0" />
<PackageReference Include="xunit">
<Version>2.3.1</Version>
</PackageReference>
</ItemGroup>
Take a look at this article for a full explanation.