Please refer to the post here

You will see that you can use naming convention (which is why the methods with Get in the name work), or you can explicitly specify the HTTP method for an action by decorating the action with the correct HTTP attribute.

Routing in ASP.NET Web API

If you are familiar with ASP.NET MVC, Web API routing is very similar to MVC routing. The main difference is that Web API uses the HTTP verb, not the URI path, to select the action. You can also use MVC-style routing in Web API. This article does not assume any knowledge of ASP.NET MVC.

If you self-host Web API, you must set the routing table directly on the HttpSelfHostConfiguration object. For more information, see Self-Host a Web API.

The reason for using "api" in the route is to avoid collisions with ASP.NET MVC routing. That way, you can have "/contacts" go to an MVC controller, and "/api/contacts" go to a Web API controller. Of course, if you don‘t like this convention, you can change the default route table.

Once a matching route is found, Web API selects the controller and the action:

• To find the controller, Web API adds "Controller" to the value of the {controller} variable.
• To find the action, Web API looks at the HTTP verb, and then looks for an action whose name begins with that HTTP verb name.

           For example, with a GET request, Web API looks for an action prefixed with "Get", such as "GetContact" or "GetAllContacts".

           This convention applies only to GET, POST, PUT, DELETE, HEAD, OPTIONS, and PATCH verbs.

           You can enable other HTTP verbs by using attributes on your controller. We‘ll see an example of that later.

• Other placeholder variables in the route template, such as {id}, are mapped to action parameters.