Adding a form to a Legacy code solution?

I've created a solution as a CLASS LIBRARY (no forms) and dumped in some legacy code which compiled without incident.

The legacy code used to work with a DIALOG screen in order to ask the user to enter an employee number.

I've added a WINDOWS FORM to the solution and added Systems.Windows.Forms and System.Drawing as references to the solution in order to incorporate the form.

In my legacy code I've added the line:

invoke my-empChange::ShowDialog(self)

in order to fire up the form however I'm getting the message: COBCH089: Could not find method 'ShowDialog' with this signature

Which other 'references' & namespaces do I need to incorporate into the solution so that the program will compile & run?