How to show WebApi OAuth token endpoint in Swagger

ApiExplorer won't be automatically generating any info for your endpoint so you'll need to add a custom DocumentFilter in order to manually describe the token endpoint.

There's an example of this at https://github.com/domaindrivendev/Swashbuckle/issues/332 :

class AuthTokenOperation : IDocumentFilter
{
    public void Apply(SwaggerDocument swaggerDoc, SchemaRegistry schemaRegistry, IApiExplorer apiExplorer)
    {
        swaggerDoc.paths.Add("/auth/token", new PathItem
        {
            post = new Operation
            {
                tags = new List<string> { "Auth" },
                consumes = new List<string>
                {
                    "application/x-www-form-urlencoded"
                },
                parameters = new List<Parameter> {
                    new Parameter
                    {
                        type = "string",
                        name = "grant_type",
                        required = true,
                        @in = "formData"
                    },
                    new Parameter
                    {
                        type = "string",
                        name = "username",
                        required = false,
                        @in = "formData"
                    },
                    new Parameter
                    {
                        type = "string",
                        name = "password",
                        required = false,
                        @in = "formData"
                    }
                }
            }
        });
    }
}

httpConfig.EnableSwagger(c =>
{
    c.DocumentFilter<AuthTokenOperation>();
});

If anyone wondered how to add a response body to this operation, here's updated Ruaidhri's code:

class AuthTokenOperation : IDocumentFilter
{
    public void Apply(SwaggerDocument swaggerDoc, SchemaRegistry schemaRegistry, IApiExplorer apiExplorer)
    {
        swaggerDoc.paths.Add("/token", new PathItem
        {
            post = new Operation
            {
                tags = new List<string> { "Auth" },
                consumes = new List<string>
                    {
                        "application/x-www-form-urlencoded"
                    },
                parameters = new List<Parameter> {
                    new Parameter
                    {
                        type = "string",
                        name = "grant_type",
                        required = true,
                        @in = "formData"
                    },
                    new Parameter
                    {
                        type = "string",
                        name = "username",
                        required = false,
                        @in = "formData"
                    },
                    new Parameter
                    {
                        type = "string",
                        name = "password",
                        required = false,
                        @in = "formData"
                    }
                },
                responses = new Dictionary<string, Response>()
                {
                    {
                        "200",
                        new Response {schema = schemaRegistry.GetOrRegister(typeof(OAuthTokenResponse))}
                    }
                }
            }
        });
    }
}

class OAuthTokenResponse
{
    [JsonProperty("access_token")]
    public string AccessToken { get; set; }

    [JsonProperty("token_type")]
    public string TokenType { get; set; }

    [JsonProperty("expires_in")]
    public long ExpiresIn { get; set; }

    [JsonProperty("userName")]
    public string Username { get; set; }

    [JsonProperty(".issued")]
    public DateTime Issued { get; set; }

    [JsonProperty(".expires")]
    public DateTime Expires { get; set; }
}