how to set connectionString for connect Database inside App_Data in Asp.net Core

Question

Old Project
Web.config

Following ConnectionString use for App_Data Database Connection

  <add name="MaContext" connectionString="Data Source=.\SQLEXPRESS;

 AttachDbFilename=|DataDirectory|MaintenanceDB.mdf;Integrated 
 Security=True;User Instance=True" providerName="System.Data.SqlClient" />  

Now I am using Asp.net Core With .NET 5

appsettings.json

 "ConnectionStrings": {

  "BCAContext": "Data Source=.\\SQLEXPRESS;   AttachDbFilename=|DataDirectory|\\MaintenanceDB.mdf;

  Integrated Security=True;User Instance=True;"
  }

Error