Here is an example of how you can modify the `Add` method to use the HTTP POST method instead of GET:

```csharp
[HttpPost]
[Route("api/calculator/add")]
public IHttpActionResult Add([FromBody] CalculatorInput input)
{
    int result = input.Num1 + input.Num2;
    return Ok(result);
}
```

You'll also need to define a `CalculatorInput` class to represent the input data for the calculator methods. Here's an example:

```csharp
public class CalculatorInput
{
    public int Num1 { get; set; }
    public int Num2 { get; set; }
}
```

With these changes, you can now send a POST request to the `/api/calculator/add` endpoint with a JSON body containing the `Num1` and `Num2` values, like this:

```json
{
    "Num1": 3,
    "Num2": 4
}
```

The API will then return the result of adding these two numbers together.

You can make similar changes to the other calculator methods to use the HTTP POST method as well.
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