1. open the %AppData%\NuGet\NuGet.Config file;

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
    <add key="FastReportRepository" value="C:\Program Files\FastReports\FastReport.Net Trial\Nugets\Core3" />
  </packageSources>
  <packageRestore>
    <add key="enabled" value="True" />
    <add key="automatic" value="True" />
  </packageRestore>
  <bindingRedirects>
    <add key="skip" value="False" />
  </bindingRedirects>
  <packageManagement>
    <add key="format" value="0" />
    <add key="disabled" value="False" />
  </packageManagement>
</configuration>


2. from package manager console:
Install-Package C:\Program Files\FastReports\FastReport.Net Trial\Nugets\Core3\FastReport.Core3.2021.1.0-demo.nupkg
Install-Package C:\Program Files\FastReports\FastReport.Net Trial\Nugets\Core3\FastReport.Core3.Web.2021.1.0-demo.nupkg