You can use a server side method to 'read' the script src file from the ASP.Net server. You would perform an HTTP GET using something like System.Net.Http.HttpClient. This is effective if you are retrieving JSON data or some other type of data to process on the server.

However, the C# backend would not process JavaScript instructions. For that you would need a server side JavaScript engine such as Node, or the client's web browser.

Including the JavaScript file in the View will allow the client's JavaScript engine to handle processing. But as you have implied, nothing in the View is hidden from the client.

You can create an Action that loads and returns the script, then reference that Action in your html, eg:

<script src='@Url.Action("LoadYandex")'>

and

public ActionResult LoadYandex() {
    // HttpClient to get the file
    return Content(file, "text/javascript");
}

Edit more detailed Action example, basic "get file from server and return contents". Look for more detailed SO answers to include error checking etc

public async Task<ActionResult> LoadYandex()
{
    var url = "https://api-maps.yandex.ru/2.1/?apikey=xxxxxxxxxx&lang=ru_RU"; 

    var httpClient = new HttpClient();
    var response = await httpClient.GetAsync(url);
    var contents = await response.Content.ReadAsStringAsync();

    return Content(contents, "text/javascript");
}