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Hi,
thanks for help. Although it's now a long time since I originally asked a question, I needed this again and I decided to write here my solution for anyone that would be in same dilemma. The answer was is almost correct and it pointed me in the right direction.

I forgot to answer it last year, because I solved this some other way and I totally forgot about this question.

Because I was using [mb_user_profile_info id="rwmb-user-info"] shortcode, I needed to create a filter like this:

add_filter( 'rwmb_profile_info_fields', function( $fields ) {
    error_log(print_r($fields, true));
    $fields['password']['attributes'] = [
        'title' => 'Username required text'
    ];
    $fields['password2']['attributes'] = [
        'title' => 'Password required text'
    ];

    return $fields;
});

Thank you, I saw this thread but I can't figure it out. If I use [mb_user_profile_login], how can I add "validation" part to the field group settings?
I can see that there is a "rwmb_profile_login_fields" filter, but this is only for changing fields, not adding to the field group settings?