Hi,
maybe you try it with this
https://www.advancedcustomfields.com/resources/acf-prepare_field/
function my_acf_prepare_field( $field ) {
$field['value']['url'] = "https";
return $field;
}
add_filter('acf/prepare_field/key=field_5c6691766a1c2', 'my_acf_prepare_field');
You only need to validate, if there is already a “good” url saved.
Or you take a look here, and find yourself another filter 🙂
https://www.advancedcustomfields.com/resources/#filters