Set form as Parent throw exception "Top-level control cannot be added to a control"

Best way would be to use EP.ShowDialog(this) and later use Owner property.

You need the EP.TopLevel property to be set to false. It will let you to set a parent to it.

Further reading.

In case you only want to access variables and controls of another form, then maybe you can reach it in other ways, not trough a Parent relationship.