https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.form.addownedform?view=netframework-4.8.1&viewFallbackFrom=windowsdesktop-10.0
Form.AddOwnedForm(Form) Method (System.Windows.Forms) | Microsoft Learn
Adds an owned form to this form.
windows formsmethodsystemmicrosoftlearn