
In the screenshot you can see the Post Object in ACF and Field Name is “region_id”.
That drop down ( Region) I have to auto select or populate based on other field ( Country) .

You need to put the code either in function.php or using custom snippet plugins.
add_filter('acf/load_field/name=region_id', function($field) {
// Check if we are on a valid post edit screen
$post_id = $_GET['post'];
// Get the value of the 'p_country' field for the current post
$p_country_value = get_field('p_country', $post_id);
if ($p_country_value === 'New Zealand') {
$field['default_value'] = array(46627);
}
if ($p_country_value === 'United States') {
$field['default_value'] = array(46628);
}
if ($p_country_value === 'Australia') {
$field['default_value'] = array(46629);
}
$field['instructions'] = $p_country_value;
return $field;
}, PHP_INT_MAX);
