mvc api security

Assume I have super simple MVC API. Our customer will use html jquery to consume the API.
I want to add one token per customer. for example,


customer A token: asdfasdfasfasfasd
customer B token: 23423423423234dsf


1. If the customer's webpage put above token there to consume my API, it has security concern because I can see the token in view source.
2. What is the simple way to implment API with token? I only have 2 to 5 API so I do not need very complex one.

[System.Web.Http.HttpGet]
        public string Get(string firstname)
	{
return firstname;

}