private Dialog<LoginInfo> createLoginDialog() {
    Dialog<LoginInfo> dlg = new Dialog<>();
    dlg.setHeaderText("Please enter login info");
    dlg.getDialogPane().getButtonTypes().addAll(ButtonType.OK, ButtonType.CANCEL);

    GridPane grid = new GridPane();
    grid.setHgap(5);
    grid.setVgap(5);
    grid.add(new Label("Username: "), 0,0);
    grid.add(new Label("Password: "), 0,1);
    TextField uname = new TextField();
    PasswordField pass = new PasswordField();
    grid.add(uname, 1,0);
    grid.add(pass, 1,1);

    // The content may be created using an FXML, of course.
    dlg.getDialogPane().setContent(grid);
    dlg.setResultConverter(buttonType -> {
        if (buttonType == ButtonType.OK)
            return new LoginInfo(uname.getText(), pass.getText());
        else
            return null;
    });
    return dlg;
}

public class LoginInfo {
    private final String user;
    private final String password;

    // TODO: Add getters and setters for fields...

    public LoginInfo(String user, String password) {
        this.user = user;
        this.password = password;
    }
}

Dialog<LoginInfo> loginDialog = createLoginDialog();
Optional<LoginInfo> info = loginDialog.showAndWait();
if (info.isPresent()) {
    // Your login logic here
} else {
    // Your cancel logic here
}