Java adding components to JOptionPane

I'm trying create a custom panel for a simple data entry dialog. I've created a custom panel which I add my labels and text fields. I'm trying to add it to a JOptionPane for display.

When I call it, my components do not display but the JOptionPane buttons do. What is the correct way of doing this?

Here is where I create my custom panel and call JOptionPane:

    public class ListenCustEdit implements ActionListener {
    @Override
    @SuppressWarnings("empty-statement")
    public void actionPerformed(ActionEvent e) {
        TestPanel panel = new TestPanel();
        int input = JOptionPane.showConfirmDialog(frame, panel, "Edit Customer:"
                        ,JOptionPane.OK_CANCEL_OPTION, JOptionPane.PLAIN_MESSAGE);
        if (input == 0) {
            // OK
            JOptionPane.showMessageDialog(frame,"Changes savewd");
        } else {
            // Cancel
            JOptionPane.showMessageDialog(frame, "No changes were saved");
        }
    }
}

Here is my custom panel class:

public class TestPanel extends JPanel {

JTextField custIdTextField;
JTextField companyTextField;
JTextField firstNameTextField;
JTextField lastNameTextField;

public TestPanel() {
    initUI();
}

public final void initUI() {
    
    // create the panel and set the layout
    JPanel main = new JPanel();
    main.setLayout(new GridLayout(4,4));
    
    // create the labels
    JLabel custIdLabel = new JLabel("Cust Id: ");
    JLabel companyLabel = new JLabel("Company: ");
    JLabel firstNameLabel = new JLabel("First Name: ");
    JLabel lastNameLabel = new JLabel("Last Name: ");
    
    // create the text fields
    custIdTextField = new JTextField();
    companyTextField = new JTextField();
    firstNameTextField = new JTextField();
    lastNameTextField = new JTextField();
    
    // add componets to panel
    main.add(custIdLabel);
    main.add(custIdTextField);
    main.add(companyLabel);
    main.add(companyTextField);
    main.add(firstNameLabel);
    main.add(firstNameTextField);
    main.add(lastNameLabel);
    main.add(lastNameTextField);
}

You need to add the main panel to your TestPanel.

public final void initUI() {
    // ...
    add(main);
}