Simple pop-up using ncurses form and menu library (not CDK) in C. This is a bit more oriented towards easy integration rather than a stand-alone example.

popup.c

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <string.h>

#include "popup.h"

static WINDOW *win_body;
static WINDOW *win_form;
static WINDOW *win_menu;
static bool is_on_button;
static MENU *popup_menu;
static ITEM **popup_items;

extern FORM *popup_form;
extern FIELD **popup_fields;
extern struct actions **button_actions;

void popup_new(int rows, int cols, int posy, int posx, int nb_buttons,
		char **requests, int nb_fields)
{
	int i, cury = 0, curx = 1, tmp;
	WINDOW *inner;

	win_body = newwin(rows, cols, posy, posx);
	assert(win_body != NULL);
	box(win_body, 0, 0);

	popup_items = malloc(sizeof(ITEM *) * (nb_buttons+1));
	assert(popup_items);

	for (i = 0; i < nb_buttons; i++) {
		popup_items[i] = new_item(button_actions[i]->key, "");
		assert(popup_items[i] != NULL);
	}

	popup_items[i] = NULL;
	popup_menu = new_menu(popup_items);
	win_menu = derwin(win_body, 3, cols-2, rows-4, 1);
	assert(popup_menu != NULL && win_menu != NULL);
	box(win_menu, 0, 0);
	set_menu_win(popup_menu, win_menu);
	set_menu_format(popup_menu, 1, nb_buttons);
	tmp = popup_menu->fcols * (popup_menu->namelen + popup_menu->spc_rows);
	tmp--;
	inner = derwin(win_menu, 1, tmp, 1, (cols-3-tmp)/2);
	assert(inner != NULL);
	set_menu_sub(popup_menu, inner);
	set_menu_mark(popup_menu, "");

	popup_fields = malloc(sizeof(FIELD *) * (nb_fields+1));
	assert(popup_fields);

	for (i = 0; i < nb_fields; i++) {
		popup_fields[i] = new_field(1, 10, cury, curx, 0, 0);
		assert(popup_fields[i] != NULL);
		set_field_buffer(popup_fields[i], 0, requests[i]);

		if (i % 2 == 1) {
			cury = cury+1;
			curx = 1;
			field_opts_on(popup_fields[i], O_ACTIVE);
			field_opts_on(popup_fields[i], O_EDIT);
			set_field_back(popup_fields[i], A_UNDERLINE); 
		} else {
			curx = 20;
			field_opts_off(popup_fields[i], O_ACTIVE);
			field_opts_off(popup_fields[i], O_EDIT);
		}
	}

	popup_fields[i] = NULL;
	popup_form = new_form(popup_fields);
	assert(popup_form != NULL);
	win_form = derwin(win_body, rows-5, cols-2, 1, 1);
	box(win_form, 0, 0);
	assert(popup_form != NULL && win_form != NULL);
	set_form_win(popup_form, win_form);
	inner = derwin(win_form, popup_form->rows+1, popup_form->cols+1, 1, 1);
	assert(inner != NULL);
	set_form_sub(popup_form, inner);

	assert(post_form(popup_form) == E_OK);
	assert(post_menu(popup_menu) == E_OK);
	is_on_button = true;
	pos_menu_cursor(popup_menu);
}

void popup_delete(void)
{
	int i;

	unpost_form(popup_form);
	unpost_menu(popup_menu);

	for (i = 0; popup_fields[i] != NULL; i++) {
		free_field(popup_fields[i]);
	}

	for (i = 0; popup_items[i] != NULL; i++) {
		free_item(popup_items[i]);
	}

	free_menu(popup_menu);
	free_form(popup_form);
	delwin(win_form);
	delwin(win_menu);
	delwin(win_body);
	win_body = NULL;
	button_actions = NULL;
}

static void driver_buttons(ITEM *item)
{
	const char *name = item_name(item);
	int i = 0;

	while (button_actions[i] && strcmp(button_actions[i]->key, name) != 0)
		i++;

	if (button_actions[i])
		button_actions[i]->func();
}

static void switch_to_buttons(void)
{
	// Those 2 lines allow the field buffer to be set
	form_driver(popup_form, REQ_PREV_FIELD);
	form_driver(popup_form, REQ_NEXT_FIELD);

	menu_driver(popup_menu, REQ_FIRST_ITEM);
	is_on_button = true;
	set_menu_fore(popup_menu, A_REVERSE); // "show" the button
}

void popup_driver(int ch)
{
	switch (ch) {
		case KEY_DOWN:
			if (is_on_button)
				break;

			if (popup_form->current == popup_fields[popup_form->maxfield-1])
				switch_to_buttons();
			else
				form_driver(popup_form, REQ_NEXT_FIELD);
			break;

		case KEY_UP:
			if (is_on_button) {
				is_on_button = false;
				set_menu_fore(popup_menu, A_NORMAL); // "hide" the button
			} else
				form_driver(popup_form, REQ_PREV_FIELD);
			break;

		case KEY_LEFT:
			if (is_on_button)
				menu_driver(popup_menu, REQ_LEFT_ITEM);
			else
				form_driver(popup_form, REQ_LEFT_FIELD);
			break;

		case KEY_RIGHT:
			if (is_on_button)
				menu_driver(popup_menu, REQ_RIGHT_ITEM);
			else
				form_driver(popup_form, REQ_RIGHT_FIELD);
			break;

		case 10:
			if (!is_on_button)
				switch_to_buttons();
			else
				driver_buttons(current_item(popup_menu));

			break;

		default:
			if (!is_on_button)
				form_driver(popup_form, ch);

			break;

	}

	if (is_on_button)
		pos_menu_cursor(popup_menu);
	else
		pos_form_cursor(popup_form);

	wrefresh(win_body);
}

void popup_refresh(void)
{
	wrefresh(win_body);
}

bool popup_exists(void)
{
	return (win_body != NULL);
}

popup.h

#ifndef __CONNMAN_POPUP_H
#define __CONNMAN_POPUP_H

#include <ncurses/form.h>
#include <ncurses/menu.h>

#ifdef __cplusplus
extern "C" {
#endif

struct actions {
	char *key; // Button name
	void (*func)(); // Execute on 'return'
};

void popup_new(int rows, int cols, int posy, int posx, int nb_buttons,
		char **requests, int nb_fields);

void popup_delete(void);

void popup_refresh(void);

bool popup_exists(void);

void popup_driver(int ch);

#ifdef __cplusplus
}
#endif

#endif

using_popup.c

/*
 * clang -o test using_popup.c popup.c -ltinfo -lform -lmenu -lncurses -Wall -Werror -g
 */
#include <stdlib.h>
#include "popup.h"

FORM *popup_form;
FIELD **popup_fields;
struct actions **button_actions;

void action_btn_ok(void)
{
	int i;

	mvprintw(LINES-2, 1, "[*] OK: ");

	for (i = 0; i < popup_form->maxfield; i++) {
		printw("%s", field_buffer(popup_fields[i], 0));

		if (field_opts(popup_fields[i]) & O_ACTIVE)
			printw("\t");
	}

	refresh();
}

void action_btn_quit(void)
{
	mvprintw(LINES-2, 1, "[*] QUIT: F1 to quit");
	refresh();
}

int main()
{
	char *requests[] = { "Password:", "pass", "Id:", "id" };
	int ch;
	struct actions act_ok, act_quit;

	act_ok.key = "OK";
	act_ok.func = action_btn_ok;
	act_quit.key = "QUIT";
	act_quit.func = action_btn_quit;

	button_actions = malloc(sizeof(struct action *) * 3);
	button_actions[0] = &act_ok;
	button_actions[1] = &act_quit;
	button_actions[2] = NULL;

	initscr();
	noecho();
	cbreak();
	keypad(stdscr, TRUE);

	popup_new(24, 80, (LINES-25)/2, (COLS-81)/2, 2, requests, 4);

	refresh();
	popup_refresh();

	while((ch = getch()) != KEY_F(1))
		popup_driver(ch);
	
	popup_delete();
	endwin();

	free(button_actions[0]);
	free(button_actions[1]);
	free(button_actions);
	return 0;
}