Thought this might help someone in the future.
If you know the property name already, you can do something like the following:
[HttpPost]
[Route("myRoute")]
public object SomeApiControllerMethod([FromBody] dynamic args){
var stringValue = args.MyPropertyName.ToString();
//do something with the string value. If this is an int, we can int.Parse it, or if it's a string, we can just use it directly.
//some more code here....
return stringValue;
}