ASP.NET Core API: Authentication popup is not showing up in Swagger UI

Assuming you have some endpoints that protected with [Authorize] attribute (can also be put on the controller).

[Route("")]
public class HelloController : ControllerBase
{
    [Authorize]
    [HttpGet("secure")]
    public IActionResult GetSomethingPrivate()
    {
        return Ok("secret");
    }

    [HttpGet("public")]
    public IActionResult GetSomethingPublic()
    {
        return Ok("hey");
    }
}

You need to define a security scheme suitable for your needs. But do not require it globally, instead add it inside an operation filter. Here I've added a simple token auth:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();
    services.AddSwaggerGen(c =>
    {
        c.SwaggerDoc("v1", new OpenApiInfo { Title = "ApiPlayground", Version = "v1" });
        c.AddSecurityDefinition("token", new OpenApiSecurityScheme
        {
            Type = SecuritySchemeType.ApiKey,
            In = ParameterLocation.Header,
            Name = HeaderNames.Authorization,
            Scheme = "Bearer"
        });
        // dont add global security requirement
        // c.AddSecurityRequirement(/*...*/);
        c.OperationFilter<SecureEndpointAuthRequirementFilter>();
    });
}

And here's the operation filter which references the token auth scheme we've just created. It checks if the endpoint needs authentication, then adds the requirement.

internal class SecureEndpointAuthRequirementFilter : IOperationFilter
{
    public void Apply(OpenApiOperation operation, OperationFilterContext context)
    {
        if (!context.ApiDescription
            .ActionDescriptor
            .EndpointMetadata
            .OfType<AuthorizeAttribute>()
            .Any())
        {
            return;
        }

        operation.Security = new List<OpenApiSecurityRequirement>
        {
            new OpenApiSecurityRequirement
            {
                [new OpenApiSecurityScheme
                {
                    Reference = new OpenApiReference { Type = ReferenceType.SecurityScheme, Id = "token" }
                }] = new List<string>()
            }
        };
    }
}

When you run the app, it works as you expect:

So does the auth popup:

Bonus: using basic auth

Define a new security scheme with following values:

public void ConfigureServices(IServiceCollection services)
{
    // ...
    services.AddSwaggerGen(c =>
    {
        // ...
        // basic auth scheme (username + password)
        c.AddSecurityDefinition("basic", new OpenApiSecurityScheme
        {
            Type = SecuritySchemeType.Http,
            Scheme = "basic"
        });
        // dont add global security requirement
        // c.AddSecurityRequirement(/*...*/);
        c.OperationFilter<SecureEndpointAuthRequirementFilter>();
    });
}

Then update the operation filter to reference basic auth scheme:

internal class SecureEndpointAuthRequirementFilter : IOperationFilter
{
    public void Apply(OpenApiOperation operation, OperationFilterContext context)
    {
        if (!context.ApiDescription
            .ActionDescriptor
            .EndpointMetadata
            .OfType<AuthorizeAttribute>()
            .Any())
        {
            return;
        }

        operation.Security = new List<OpenApiSecurityRequirement>
        {
            new OpenApiSecurityRequirement
            {
                [new OpenApiSecurityScheme
                {
                    Reference = new OpenApiReference
                    {
                        Type = ReferenceType.SecurityScheme, 
                        Id = "basic" // <-- changed "token" -> "basic"
                    }
                }] = new List<string>()
            }
        };
    }
}

here's how the auth popup looks:

After logging in, requests include the correct Authorization header.

In my case, I am using JWT Token Authentication with .NET Core API. I Configure the swagger with the authorization token using the below code. This code will add global security requirements.

In Startup Class ConfigureServices Method.

        //Swagger Configuration

        services.AddSwaggerGen(options =>
        {
            options.SwaggerDoc("v1", new Microsoft.OpenApi.Models.OpenApiInfo
            {
                Title = "API",
                Version = "v2",
                Description = "Your Api Description"
            });
            options.AddSecurityDefinition("Bearer", new OpenApiSecurityScheme
            {
                Description = "JWT Authorization header using the Bearer scheme (Example: 'Bearer 12345abcdef')",
                Name = "Authorization",
                In = ParameterLocation.Header,
                Type = SecuritySchemeType.ApiKey,
                Scheme = "Bearer"
            });
            options.AddSecurityRequirement(new OpenApiSecurityRequirement
            {
                {
                    new OpenApiSecurityScheme
                    {
                        Reference = new OpenApiReference
                        {
                            Type = ReferenceType.SecurityScheme,
                            Id = "Bearer"
                        }
                    },
                    Array.Empty<string>()
                }
            });
        });

And In Configure Method

        app.UseSwagger();

        app.UseSwaggerUI(options =>
        {
            options.SwaggerEndpoint("/swagger/v1/swagger.json", "API");
        });

After running the API project Authorize button will appear on the right side. On Click the authorize button Authorization popup open and then pass the token in the text box with 'Bearer token'.

Authorization working fine for me.