API key in header with swashbuckle

Update 2021-09-15:

As already noted in Justin Greywolf's comment.

The "In" and "Type" properties have been changed from a string to the ParameterLocation and SecuritySchemeType enums:

services.AddSwaggerGen(c =>{
    c.SwaggerDoc("v1", new Info { Title = "[anything]", Version = "v1" });
    c.AddSecurityDefinition("[auth scheme: same name as defined for asp.net]", new ApiKeyScheme() {
        In = ParameterLocation.Header,
        Name = "X-API-KEY", //header with api key
        Type = SecuritySchemeType.ApiKey,
    });
});

Update 2019-04-10:

The paradigm has shifted to accommodate security definitions in the generated swagger.json

Source https://github.com/domaindrivendev/Swashbuckle.AspNetCore#add-security-definitions-and-requirements

services.AddSwaggerGen(c =>{
    c.SwaggerDoc("v1", new Info { Title = "[anything]", Version = "v1" });
    c.AddSecurityDefinition("[auth scheme: same name as defined for asp.net]", new ApiKeyScheme() {
        In = "header", // where to find apiKey, probably in a header
        Name = "X-API-KEY", //header with api key
        Type = "apiKey", // this value is always "apiKey"
    });

});

Original Answer

Check it out:

config
    .EnableSwagger(c =>
    {
        c.ApiKey("apiKey")
            .Description("API Key Authentication")
            .Name("X-ApiKey")
            .In("header");
        c.SingleApiVersion("v1", "My API");

    })
    .EnableSwaggerUi(c => {
        c.EnableApiKeySupport("X-ApiKey", "header");
    })

You'll have to inject a custom index.html based on the original (as described here) and change the following line in the function addApiKeyAuthorization:

var apiKeyAuth = new SwaggerClient.ApiKeyAuthorization("X-ApiKey", key, "header");