It was possible to implement processing of spaces and hiding passwords behind spaces. Thus, it was possible to check data during input and hide the entered data.

In the future, correction of incorrect input using other keys is necessary.

C language:

#include <form.h>
#include <stdio.h>

#define SIZEPASS 20

int main() {
    initscr();
    raw();
    noecho();
    curs_set(2);
    WINDOW *win = newwin(5, 40, 10, 10);
    keypad(win, TRUE);
    box(win, 0, 0);
    wrefresh(win);

    FIELD *fields[2];
    fields[0] = new_field(1, SIZEPASS, 2, 2, 0, 0);
    fields[1] = NULL;
    set_field_type(fields[0], TYPE_REGEXP, "^\\\\*[0-9A-Za-z\\\\*]* *$");
    set_field_opts(fields[0], O_VISIBLE | O_PUBLIC | O_EDIT | O_ACTIVE);
    set_field_back(fields[0], A_UNDERLINE);

    FORM *form = new_form(fields);
    set_form_win(form, win);
    set_form_sub(form, derwin(win, 3, 38, 1, 1));
    post_form(form);
    wrefresh(win);

    char password[SIZEPASS] = {};
    int ch, status, count = 0, stop = 0;

    while (!stop && (ch = wgetch(win)) != KEY_F(2)) {
        switch (ch) {
            case KEY_BACKSPACE:
                form_driver(form, REQ_DEL_PREV);
                if (count)
                    password[--count] = '\0';
                break;
            case 32:
                break;
            case 10:
            case KEY_ENTER:
                stop = 1;
                break;
            default:
                status = form_driver(form, ch);
                if( status == E_OK ) {
                    status = form_driver(form, REQ_VALIDATION);
                }
                if (status == E_INVALID_FIELD) {
                    form_driver(form, REQ_DEL_PREV);
                } else {
                    password[count++] = (char)ch;
                    password[count] = '\0';
                    form_driver(form, REQ_DEL_PREV);
                    form_driver(form, '*');
                }
        }
    }

    unpost_form(form);
    free_form(form);
    free_field(fields[0]);
    endwin();

    printf("Password: %s\n", password);

    return 0;
}

D language:

module source.app;

import std.conv;
import std.string;
import std.stdio;

public import deimos.ncurses;
public import deimos.form;
public import core.stdc.locale;

int main(string[] args) {
    setlocale(LC_CTYPE,"");
    initscr();
    raw();
    noecho();
    curs_set(2);
    WINDOW *win = newwin(5, 40, 10, 10);
    keypad(win, TRUE);
    box(win, 0, 0);
    wrefresh(win);

    FIELD*[2] fields;
    fields[0] = new_field(1, 20, 2, 2, 0, 0);
    fields[1] = null;
    set_field_type(fields[0], TYPE_REGEXP, `^\**[0-9A-Za-z\*]* *$`.ptr);
    set_field_opts(fields[0], O_VISIBLE | O_PUBLIC | O_EDIT | O_ACTIVE);
    set_field_back(fields[0], A_UNDERLINE);

    FORM *form = new_form(cast(FIELD**)fields);
    set_form_win(form, win);
    set_form_sub(form, derwin(win, 3, 38, 1, 1));
    post_form(form);
    wrefresh(win);

    string password;
    bool stop = false;
    int ch, status;

    while (!stop && (ch = wgetch(win)) != KEY_F(2)) {
        switch (ch) {
            case KEY_BACKSPACE:
                form_driver(form, REQ_DEL_PREV);
                if (password.length)
                    password = password[0 .. $ - 1];
                break;
            case 32:
                break;
            case 10:
            case KEY_ENTER:
                stop = true;
                break;
            default:
                status = form_driver(form, ch);
                if( status == E_OK ) {
                    status = form_driver(form, REQ_VALIDATION);
                }
                if (status == E_INVALID_FIELD) {
                    form_driver(form, REQ_DEL_PREV);
                } else {
                    password ~= ch.to!char;
                    form_driver(form, REQ_DEL_PREV);
                    form_driver(form, '*'.to!int);
                }
                break;
        }
    }

    unpost_form(form);
    free_form(form);
    free_field(fields[0]);
    endwin();

    writeln("Password: " ~ password);

    return 0;
}