To build a fetch call to pass a value to a method that expects an object in ASP.NET MVC, you can follow these steps:

1. Define the object: First, you need to define the object that you want to pass to the method. The object should have the required properties that the method expects.
2. Create a JSON string: Convert the object into a JSON string using the JSON.stringify() method.
3. Create the fetch call: Use the fetch() function to make a network request to the server. Set the method, headers, and body of the request according to the API endpoint you’re calling.
4. Set the body: Set the body property of the request to the JSON string you created in step 2.
5. Set the headers: Set the Content-Type header to application/json to indicate that the request body is in JSON format.
6. Handle the response: Once the request is completed, you can handle the response from the server using the .then() method. You can parse the response as JSON using the response.json() method and then pass the resulting object to the method that expects it.

Here’s an example of how this could look in code:

const obj = { id: 1, name: 'John Doe' };
const json = JSON.stringify(obj);

fetch('/api/endpoint', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  body: json
})
.then(response => response.json())
.then(data => {
  // Pass the returned object to a method that expects it
  myMethod(data);
})
.catch(error => {
  console.error('Error:', error);
});

In ASP.NET MVC, you can create a controller action that accepts an object as a parameter and then use the [HttpPost] attribute to indicate that it should be called when a POST request is made to a specific URL.

For example:

[HttpPost]
public ActionResult MyAction(MyObject obj)
{
    // Do something with the object
    return View();
}

In this example, MyObject is the class that defines the object that we want to pass to the controller action. When the fetch call is made to the /api/endpoint URL, it will call the MyAction method on the controller and pass in the deserialized MyObject instance.