Web API method with FromUri attribute is not working in Postman

There are 2 major issues with your post, firstly your controller (due to [FromUri] binding) is specifying that the arguments need to be passed as Query String parameters and not Http Header values.

The second issue is that you have defined two complex type parameters that you want to obtain the values form the URI, this is not supported.

How to pass in Uri complex objects without using custom ModelBinders or any serialization?
This is a great writeup on how to fully exploit the [FromUriAttribute][2] up to ASP.Net Core 2.2, many of the principals apply to the FromQueryAttribute which is still used the current in ASP.Net 6.

We can use [FromUri] to bind multiple primitive typed parameters, or we can bind 1 single complex typed parameter. You cannot combine the two concepts, the reason for this is that when a complex type is used ALL of the query string arguments are bound to that single complex type.

So your options are to create a new complex type that combines all the properties from both types, or declare all the properties of both types as primitive parameters to the method:

http://localhost:29844/api/bindings/SumNumbers1?First=3&Second=2&Add=True&Double=False

http://localhost:29844/api/bindings/SumNumbers2?First=3&Second=2&Add=True&Double=False

[HttpGet]
[HttpPost]
public int SumNumbers1([FromUri] int First, [FromUri] int Second, [FromUri] bool Add, [FromUri] bool Double)
{
    int result = Add ? First + Second : First - Second;
    return Double ? result * 2 : result;
}

[HttpGet]
[HttpPost]
public int SumNumbers2([FromUri] SumRequest req)
{
    int result = req.Add ? req.First + req.Second : req.First - req.Second;
    return req.Double ? result * 2 : result;
}

public class SumRequest
{
    public int First { get; set; }
    public int Second { get; set; }
    public bool Add { get; set; }
    public bool Double { get; set; }
}

It is also technically possible to use a nested structure, where you wrap the multiple complex with a single outer complex type. Depending on your implementation and host constraints you may need additional configuration to support using . in the query parameters, but a nested or wrapped request implementation would look like this:

http://localhost:29844/api/bindings/SumNumbers3?Calc.First=3&Calc.Second=2&Op.Add=True&Op.Double=False

[HttpGet]
[HttpPost]
public int SumNumbers3([FromUri] WrappedRequest req)
{
    int result = req.Op.Add ? req.Calc.First + req.Calc.Second : req.Calc.First - req.Calc.Second;
    return req.Op.Double ? result * 2 : result;
}

public class WrappedRequest
{
    public Numbers Calc { get; set; }
    public Operation Op { get; set; }
}

It is also possible to use a combination of Http Headers and query string parameters, however these are generally harder (less common) to manage from a client perspective.

It is more common with complex parameter scenarios (not to mention more REST compliant) to force the caller to use POST to access your calculation endpoint, then multiple complex types are simpler to support from both a client and API perspective.