set owner of Windows Form that exist in ClassLibrary c#

I have ClassLibrary that have a Windows Form named LookUpBox. I want to Call LookUpBox from my application and set my application as owner of that and do it as blow:

LookUpBox foo = new LookUpBox();
foo.Owner = this;
foo.ShowDialog();

But when Run my application and press Alt+Enter I see 2 form like :

I excepted with set of Owner of the foo, I see single Window with Alt+Enter. Anybody know how to solve this problem? Thanks in advance