Sample.cs

public record MyCommand([property: Identity] Guid Id, string SomeValue);

[AttributeUsage(AttributeTargets.Method)]
public class MyAttribute: Attribute {
  public MyAttribute(string something) => Something = something;
  
  public string Something {get;}
}

public static class MyCommandHandler {
[AggregateHandler]
[MyAttribute("Some Value")]
  public static Events Handle(MyCommand cmd) {
    // do work
    return [];
  }
}
 
public static class MyMiddleware {
  public void Before(MyCommand command, MyAttribute attribute) {
    // do something with the value in the attribute
  }
}