"Unwrapping" posted JSON

I'm looking for a way to "unwrap" JSON that's been posted to a MVC Core service. Let's say, I have the following method:

[HttpPost]        
public dynamic SayHello(string FirstName, string SecondName)
{
    return $"Hello {FirstName} {SecondName} !";
}

And I post in the following JSON:

{
    "FirstName":"Joe",
    "SecondName": "Bloggs"
}

I'd expect to get a response of Hello Joe Bloggs !, but I cannot seem to find an easy way to unwrap the JSON object into the properties of the method.

I know the correct solution is to have a HelloModel with those two properties in, slap on a [FromBody] attribute, but for reasons this isn't possible.