your ConnectionStrings should be something like :

 "ConnectionStrings": {"DefaultConnection": "Server={DBAddress};Database={DBName};User Id={user};Password={Pass};MultipleActiveResultSets=true"},

and your ApplicationDbContext should be something like :

public MDBContext(IConfiguration config, DbContextOptions<MDBContext> options) : base(options)
{
    appConfig = config;
}
public DbSet<testdb> Mytest { get; set; }

protected override void OnConfiguring(DbContextOptionsBuilder options)
{
    if (!options.IsConfigured)
    {
    options.UseSqlServer(appConfig.GetConnectionString("NameOFYourCNStringInAppsettings"));

    }

}

it is better to reviewed: https://learn.microsoft.com/en-us/ef/core/dbcontext-configuration/