Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();

    var key = Encoding.ASCII.GetBytes("YOUR_KEY");  
    services.AddAuthentication(x =>  
        {  
            x.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;  
            x.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;  
        })  
        .AddJwtBearer(x =>  
        {  
            x.TokenValidationParameters = new TokenValidationParameters  
            {  
                IssuerSigningKey = new SymmetricSecurityKey(key),  
                ValidateIssuer = false,  
                ValidateAudience = false,  
                ValidIssuer = "localhost",  
                ValidAudience = "localhost"  
            };  
        });  
}