Passing in any Types for a generic parameter of a function?

I have a method in C# that needs to take in an object that has generic type. The method looks like this:

public class MyTestclass {
       public void Attach(ModelObject<T> obj)
        {
            //...
        }
}

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The type T can be any type. This means the method Attach should be able to take in a ModelObject<string>, ModelObject<bool> or any other types for the generic, like:

var cls = new MyTestClass();
cls.Attach(new ModelObject<string>());
cls.Attach(new ModelObject<bool>());

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Because the type T is only used within the method Attach and no where else in the class, I don't wish to put the generic type at the class declaration. Moreover, declaring the type as a class generic will not allow me to have different types of T as the parameter for the method Attach.

What other workarounds can I use to be able to pass any types of T as the ModelObject<T> to the method Attach without having to define the generics at the class declaration?