I want add a userControl in Windows Form

Unless it is a WPF UserControl, it should be in the Toolbox.

If the UserControl is not in the same project, then you just need to add a refernce to the library where the UserControl is. Go to the project in Solutions Explorer, then right-click on the References node then click Add Reference. Find the library that the UserControl is in and add the library. Then the UserControl will be in the Toolbox.

Do you need to create a UserControl or add an existing one to a form? If you want to add an existing one, then it depends where it is. You should be able to find the UserControl in the Toolbox. If it is not in the Toolbox, then tell us if the UserControl is in a file in your project or is it in a class in a separate project. If it is a separate project, is the project a library project? If the UserControl is in a library project then you need to add a reference to it in the project you need to use it in.