How to send custom headers with requests in Swagger UI?

In ASP.NET Core 2 Web API, using Swashbuckle.AspNetCore package 2.1.0, implement a IDocumentFilter:

SwaggerSecurityRequirementsDocumentFilter.cs

using System.Collections.Generic;
using Swashbuckle.AspNetCore.Swagger;
using Swashbuckle.AspNetCore.SwaggerGen;

namespace api.infrastructure.filters
{
    public class SwaggerSecurityRequirementsDocumentFilter : IDocumentFilter
    {
        public void Apply(SwaggerDocument document, DocumentFilterContext context)
        {
            document.Security = new List<IDictionary<string, IEnumerable<string>>>()
            {
                new Dictionary<string, IEnumerable<string>>()
                {
                    { "Bearer", new string[]{ } },
                    { "Basic", new string[]{ } },
                }
            };
        }
    }
}

In Startup.cs, configure a security definition and register the custom filter:

public void ConfigureServices(IServiceCollection services)
{
    services.AddSwaggerGen(c =>
    {
        // c.SwaggerDoc(.....

        c.AddSecurityDefinition("Bearer", new ApiKeyScheme()
        {
            Description = "Authorization header using the Bearer scheme",
            Name = "Authorization",
            In = "header"
        });

        c.DocumentFilter<SwaggerSecurityRequirementsDocumentFilter>();
    });
}

In Swagger UI, click on Authorize button and set value for token.

Result:

curl -X GET "http://localhost:5000/api/tenants" -H "accept: text/plain" -H "Authorization: Bearer ABCD123456"

In ASP.NET Web API, the simplest way to pass-in a header on Swagger UI is to implement the Apply(...) method on the IOperationFilter interface.

Add this to your project:

public class AddRequiredHeaderParameter : IOperationFilter
{
    public void Apply(Operation operation, SchemaRegistry schemaRegistry, ApiDescription apiDescription)
    {
        if (operation.parameters == null)
            operation.parameters = new List<Parameter>();

        operation.parameters.Add(new Parameter
        {
            name = "MyHeaderField",
            @in = "header",
            type = "string",
            description = "My header field",
            required = true
        });
    }
}

In SwaggerConfig.cs, register the filter from above using c.OperationFilter<T>():

public static void Register()
{
    var thisAssembly = typeof(SwaggerConfig).Assembly;

    GlobalConfiguration.Configuration 
        .EnableSwagger(c =>
        {
            c.SingleApiVersion("v1", "YourProjectName");
            c.IgnoreObsoleteActions();
            c.UseFullTypeNameInSchemaIds();
            c.DescribeAllEnumsAsStrings();
            c.IncludeXmlComments(GetXmlCommentsPath());
            c.ResolveConflictingActions(apiDescriptions => apiDescriptions.First());


            c.OperationFilter<AddRequiredHeaderParameter>(); // Add this here
        })
        .EnableSwaggerUi(c =>
        {
            c.DocExpansion(DocExpansion.List);
        });
}

You can add a header parameter to your request, and Swagger-UI will show it as an editable text box:

swagger: "2.0"
info:
  version: 1.0.0
  title: TaxBlaster
host: taxblaster.com
basePath: /api
schemes:
- http

paths:

  /taxFilings/{id}:

    get:
      parameters:
      - name: id
        in: path
        description: ID of the requested TaxFiling
        required: true
        type: string
      - name: auth
        in: header
        description: an authorization header
        required: true
        type: string
      responses:
        200:
          description: Successful response, with a representation of the Tax Filing.
          schema:
            $ref: "#/definitions/TaxFilingObject"
        404:
          description: The requested tax filing was not found.

definitions:
  TaxFilingObject:
    type: object
    description: An individual Tax Filing record.
    properties:
      filingID:
        type: string
      year:
        type: string
      period:
        type: integer
      currency:
        type: string
      taxpayer:
        type: object

You can also add a security definition with type apiKey:

swagger: "2.0"
info:
  version: 1.0.0
  title: TaxBlaster
host: taxblaster.com
basePath: /api
schemes:
- http

securityDefinitions:
  api_key:
    type: apiKey
    name: api_key
    in: header
    description: Requests should pass an api_key header.

security: 
 - api_key: []

paths:

  /taxFilings/{id}:

    get:
      parameters:
      - name: id
        in: path
        description: ID of the requested TaxFiling
        required: true
        type: string

      responses:
        200:
          description: Successful response, with a representation of the Tax Filing.
          schema:
            $ref: "#/definitions/TaxFilingObject"
        404:
          description: The requested tax filing was not found.

definitions:
  TaxFilingObject:
    type: object
    description: An individual Tax Filing record.
    properties:
      filingID:
        type: string
      year:
        type: string
      period:
        type: integer
      currency:
        type: string
      taxpayer:
        type: object

The securityDefinitions object defines security schemes.

The security object (called "security requirements" in Swagger–OpenAPI), applies a security scheme to a given context. In our case, we're applying it to the entire API by declaring the security requirement a top level. We can optionally override it within individual path items and/or methods.

This would be the preferred way to specify your security scheme; and it replaces the header parameter from the first example. Unfortunately, Swagger-UI doesn't offer a text box to control this parameter, at least in my testing so far.