gistfile1.txt

function registration_form_add_country_field() {

	// Only show if secret arg is in URL.
	if ( empty( $_GET['add_field'] ) || $_GET['add_field'] != 1 ) {
		return;
	}

	$form         = GFAPI::get_form( TEST_FORM_ID );
	$new_field_id = 0;

	foreach ( $form['fields'] as $field ) {
		if ( $field->id > $new_field_id ) {
			$new_field_id = $field->id;
		}
	}

	$countries_array = [
		'Afghanistan'                       => 'AF',
		'Åland Islands'                     => 'AX',
		'Albania'                           => 'AL',
		'Algeria'                           => 'DZ',
		'Andorra'                           => 'AD'
    ];

	$choices = [];

	foreach ( $countries_array as $name => $code ) {
		$choices[] = [
			'text' => $name,
			'value' => $code
		];
	}

	$new_field_id ++;
	$field              = GF_Fields::get( 'select' );
	$field->id          = $new_field_id;
	$field->name        = 'countries';
	$field->label       = 'Country Yeeee Haaaawwww!';
	$field->placeholder = 'Select a country';
	$field->choices     = $choices;
	$form['fields'][]   = $field;
	GFAPI::update_form( $form );
}